Cannabis is a drug that comes from a plant, or a plant which is used as a drug. Cannabis use has been falling; most people in the UK (more than two out of three) have never tried cannabis, but it’s still the most popular illegal drug.

Cannabis comes in many different forms. Its users are varied too, taking the drug in different ways, for different reasons. Some people use it looking for fun and laughter, some hope for medical benefits, some want help to relax. Some people use cannabis out of habit or because they find it difficult to resist. Cannabis can deliver the positive effects its users hope for, but it doesn’t always do so, and it can cause harm too.

This page will let you know some of the risks of cannabis use that users can easily avoid, and also help you to recognise those risks you can never totally get away from if you use it. With this information, it’s easier to make informed personal decisions about whether it’s worth the risk to use cannabis at all, and if you do use it, to keep those risks as low as you can make them.

It’s hard to say how harmful the use of a drug like cannabis is ‘overall’, because it depends so much on the user, how they use it, and even the type of cannabis. Most people who try cannabis are not harmed by it at all. However, cannabis can play a very destructive role in some people’s lives, particularly if they become addicted to it. Regular users sometimes notice that cannabis is damaging their physical or mental wellbeing, others feel it holds them back from fulfilling their potential. As with any drug, the only relationship with cannabis that is 100% safe is avoiding it entirely. However, if somebody wants to enjoy cannabis and accepts the possibility of harm, there are many important choices that can reduce the risks. There are powerful choices to be made about what cannabis to use, when and how to use it.

An analysis by Drug Science which added up and compared the harm caused by 20 different drugs in the UK placed cannabis roughly in the middle; it causes more harm to society than several other illegal drugs (mostly because there are many more users) but less harm to its users than the legal drugs tobacco and alcohol, both of which have a higher potential to cause serious illness and death. This study estimated the total harm done by cannabis in 2010 in the UK. Another study by Drug Science also looked at how changing governmental legislation on cannabis can reduce harm to users and found that state control of cannabis was the best policy to minimise drug harms.

Therefore, better political choices could make cannabis safer overall.

Similarly to alcohol, cannabis can help people relax and feel pleasantly sleepy and ‘stoned’, or in another setting, people use it to socialize, feel ‘high’ and laugh. It can make people clumsy with their bodies and words. Less like alcohol, cannabis also can alter the senses in unusual ways, so music, food, films, computer games might seem different, more fascinating, and easier to feel immersed in.

Cannabis has some side effects such as drying out the mouth and eyes (which can look red and feel itchy), and generating a strong appetite; ‘the munchies’.

Especially for someone who has never taken cannabis before, the effects can be hard to predict, and can be unpleasant for some people. It’s common for people to try cannabis and not want to use it again.

Taking cannabis has the effects it has because it contains some unique drug chemicals called ‘cannabinoids’. The cannabinoid which is most important in making someone ‘stoned’ or ‘high’ is called THC. Some cannabis varieties (such as skunk) can contain much more THC than other types, and so give stronger effects or can be used in smaller amounts. However, scientists and cannabis users are also discovering that it’s not just the amount of THC that is important, but also the balance of THC with other cannabinoids, especially one called CBD. Cannabis varieties with lots of CBD (like most ‘hash’) might feel different (more relaxing, less confusing) and have a lower chance of causing harm, than cannabis varieties with little CBD (such as skunk). This is because CBD interacts with THC and reduces its “stoned” effects.

Our minds respond to cannabinoids from cannabis because we have a system in our brains called the endocannabinoid system that it can interact with. This system has a role in controlling memory, mood, appetite, sleep and other functions. The endocannabinoid system is not there to be triggered by cannabinoids from cannabis; the fact that it can be manipulated by this plant may be due to a close co-evolution of humans and the cannabis plant family.

The endocannabinoid system responds to the body’s own endocannabinoids; these are cannabinoid chemicals made in our brains. The effect of cannabinoids from cannabis on the endocannabinoid system doesn’t entirely explain how cannabis users feel and act when they are stoned. Without realising it, people learn to fit in with acting a certain way when they are stoned and around other stoned people. This helps explain why one substance, in different cultural contexts, can have such different effects.

The two main ways that people get the cannabis chemicals into their body are through the lungs, by smoking or vaporizing it, or through the gut (orally), by eating or drinking it or by holding cannabis oil under the tongue.

The effects of cannabis when smoked or vaporized appear almost immediately, reach their strongest in the first 15 minutes or so, start to fade before an hour has passed, and are nearly gone after about 3 hours, although the exact timings vary from person to person.

When used orally, the effects of cannabis take much longer to appear and rise to their peak, you might feel nothing for an hour. Then the effects can last 5 hours or even more. This time can be very different from person to person.

The under-the-tongue route produces effects roughly half way between vaping /smoking and oral use.

The commonest way of using cannabis in the UK is also the worst for your health. That is adding it to tobacco in a cigarette paper, rolling it with no filter and smoking this ‘joint’. Tobacco smoke can give you lung cancer, COPD, and other potentially deadly diseases. Tobacco is also addictive, much more so than cannabis, so smoking cannabis joints can be a ‘gateway’ to a harmful and expensive tobacco addiction. Or it might lead you to craving more joints, and so smoking more cannabis than you might otherwise. Smoking cannabis as well as tobacco might lead to more harm than smoking tobacco only.

Smoking cannabis on its own (in a pipe or bong for example, or rolled in a tobacco-free cigarette) is much less harmful. The risk of lung cancer is far lower, maybe as low as someone who doesn’t smoke at all. Also, using tobacco makes it harder to breathe deeply and freely, cannabis alone seems not to. However, burning any plant, cannabis or tobacco, makes hundreds of toxic chemicals, and a cannabis smoker is still breathing in this cloud full of little irritating particles. This can cause inflammation, coughing and wheezing (bronchitis, which goes away when a user quits). How badly cannabis affects the lungs depends, of course, on how much someone uses it. People who smoke occasionally might not notice any problem, but constant cannabis smoking might do damage.

People who want the rapid effects of inhaling cannabis without the smoke can use a vaporiser instead. Vaporisers heat up the cannabis hot enough that the cannabinoids and some other substances evaporate and can be inhaled, but not so hot that the plant material is burnt. The vapour can still make you cough, but there is some research now to show that it can cause less harm. Moreover, avoiding the chemicals that result from burning is likely to be a good move.

Cannabis can also be cooked into food (such as brownies). This avoids any risk to the lungs, but a possible downside is that the effects can take a long time to come on and can take longer for some people than others, making it much harder to judge how much to take. This can lead to taking way too much and having a horrible few hours.

The immediate effects of cannabis can be nasty but are rarely catastrophic. Let’s start with common unwanted effects, things which might sound familiar to many people who have tried the drug.

Taking too much too fast (which is easy when the cannabis is very strong) can be horrible. Someone may suddenly look and feel pale, shaky and ill, (sometimes called greening out, or pulling a whitey) they may even vomit or collapse. This effect can be a result of a rapid drop in blood pressure. It’s a good reason to smoke at a slow pace, to get a feel for the strength of the cannabis. If someone does feel ill, it usually doesn’t last long and the person may recover faster if they lie down with their feet up. Whatever the drug is, never leave someone alone who is vomiting and intoxicated, as there is a risk of choking. If they are vomiting and intoxicated make sure their airways are cleared and consider putting them in the recovery position. Combining cannabis with alcohol or other drugs creates a higher risk of feeling awful and regretting it.

Even people who have enjoyed using cannabis before can unexpectedly find that it gives them bad rather than good feelings. Sometimes people just feel overwhelmed and might need to lie down for a bit, especially if they get more stoned than they intended to. Even when taking a typical amount, the same as you might have enjoyed before or the amount that your friends are enjoying, users might find that they feel worried, suspicious, sad, confused or even feel trapped going round in circles of weird thoughts.

Usually, if someone becomes panicked, confused, or behaves oddly when intoxicated on cannabis, they will be alright given time, especially if they are reassured and guided to breathe slowly and relax. Sometimes, they will feel fine after a few minutes, sometimes they will not feel good, but will be able to hold out with some friendly support until they sober up a bit. Very very occasionally, if someone remains very frightened or paranoid, and does not seem to respond to reassurance, or their behaviour is very alarming, it may be sensible to get help, for example by calling an ambulance. This can be a difficult judgement to make, remember that the good or bad effects of cannabis usually start to decline on their own after about an hour after inhalation, or a couple of hours after eating it.

It can be common for people who are intoxicated on cannabis (especially strong ‘skunk’ strains which are high in THC and low in CBD) to say things or feel things which are just like symptoms of psychosis, for example to think other people are taking about them or being paranoid. Experiencing some psychotic symptoms whilst stoned isn’t the same as actually being psychotic. However, if you or someone else should have sobered up, but are still feeling paranoia or delusional ideas, it is important to get medical help. Extremely rarely, getting intoxicated on cannabis can trigger a mental health crisis such as a ‘psychotic episode’, which carries on even after the person should have sobered up. This can be a serious situation, and they need to have medical assistance to give them the best chance of getting better fast without any lasting effects. This usually only happens to people who already have a vulnerability to mental health problems. Most people who have a psychotic episode triggered by cannabis (or anything else) recover well over the next few days with treatment. However, having this type of reaction to cannabis even once is a strong indication that continuing to use cannabis could be very harmful. There’s more information on this in the section Will cannabis make me lose control of my mind?

Cannabis is one of the least toxic drugs. Unlike drugs such as alcohol, paracetamol, heroin or ecstasy, users do not die after accidentally taking too much. But like all drugs that distort the way you think and behave, cannabis use creates a risk of hurting yourself doing something stupid, ranging from trying to cook to trying to cross a motorway. People who drive stoned double or triple their risk of crashing, injuring and killing others as well as themselves.

Occasionally after using cannabis, people feel their heart flutter, skip beats, race or pound in their chest. These unpleasant but usually harmless feelings (called palpitations and tachycardia) can be scary, especially as cannabis can make sensations feel more intense than usual, and make anxiety worse. Of course, when someone gets scared by this effect, it can make the heart beat even harder and lead to a panic attack.

Cannabis use does put some extra stresses on the body, and there is some evidence suggesting that heart attacks and strokes are slightly more likely to happen soon after using cannabis, just as they can be triggered by exercise or sex. This is a tiny risk that is not relevant to most users, but people with health problems (like an irregular heartbeat) should always be aware that drugs might have greater risks to them.

Cannabis smokers sometimes do some unnecessary things involving breath-holding, either as part of smoking games or in the mistaken belief that holding in the smoke will get them higher, faster. Struggling to hold your breath when you want to cough or breathe out can create sudden pressure changes in the lungs, as can sucking very forcefully from a bong. These habits have led to surprising lung injuries , where air has forced its way through tiny tears in the lung and ended up in places where it shouldn’t be, but these injuries are rare.

Using cannabis when pregnant could have effects on the foetus. The baby might weigh less when it is born, and the child might have a bigger chance of having behaviour problems and doing worse at school as it grows up. More research is needed to be sure about this.

Most of the serious harm caused by cannabis use is suffered by the minority of users who use it regularly, who take it daily or nearly daily, not to the majority of users who like to have some now and then. Like alcohol or fast food, the most important thing is staying in control, not letting cannabis become an essential part of your lifestyle. Using less is the simplest and most effective way to avoid harm. Another way is by understanding the relative THC content of the cannabis you smoke. Strains with higher THC are likely to cause more damage and so minimising these strains, where possible, will go a long way to reducing negative effects.

Don’t be discouraged from cutting down by the thought “I smoke loads of cannabis; if I smoked half as much I’d still be smoking loads”. In fact, halving the amount you consume will probably take away more than half of the risk.

It used to be thought that cannabis isn’t addictive. Now it is clear that this is not true at all, about 9% of people that have used cannabis will become addicted. Some people choose to use cannabis frequently but fit it around their life and can take a break from it easily, but others can’t go without it and find that it has become a real problem in their life. People who live alone, have gone through recent difficulties like money troubles, and who use cannabis to cope with their problems are most likely to get addicted, but it could happen to any heavy user.

Addictions to different drugs have very different consequences. People who are addicted to alcohol can become very ill and even die if they are unable to drink it. People who are addicted to cannabis don’t get seriously ill without it, but might find it just as hard to quit the drug or regain control over their use. A person addicted to cannabis can become irritable and anxious if they try to go without it, lose their appetite, and feel unable to relax and sleep. These effects get considerably better after the first three days. Going to your doctor can help you access support towards finding a way to overcome cannabis addiction. People trying to quit cannabis also support each other online.

A small number of people who smoke lots of cannabis get an illness where they feel sick, have stomach aches and vomit. Mysteriously, these people usually find that very hot baths and showers help a bit, although stopping cannabis use is the only known solution. This illness is called Cannabis Hyperemesis, and in one case, the vomiting was violent enough to tear a hole in the oesophagus (food pipe).

In the UK, the thing about cannabis use that is most talked about by politicians and written about in newspapers is the fear that it can make young people become mentally ill, and maybe never recover. There is some truth and often plenty of exaggeration to these concerns. To put it in proportion, for most young people experimenting with cannabis, the risk of going on to become addicted is much higher than the risk of going on to develop serious mental illness such as psychosis. Psychosis is most clearly linked to very heavy use of cannabis like ‘skunk’, which has lots of THC in it (making it strong), but almost no CBD (a protective chemical). Using cannabis with lots of CBD in it, like most types of hash, might not be linked to psychosis at all. In fact, CBD on its own has been tried out as a treatment for psychosis, and results are promising.

For some people, this issue should matter more because the risk of cannabis provoking psychosis will be higher for them. For example, if you have a close relative who has had episodes of psychosis, or has schizophrenia, cannabis might be enough to give you a push towards developing such conditions yourself.

To understand the risk, you have to understand a little about what psychosis is and what causes it to develop (often in early adulthood). Someone with psychosis may experience paranoia (e.g. thinking people are plotting against you) delusions (e.g. thinking that you have a mission, and are being sent guidance through the radio) and hallucinations (e.g. hearing voices). Most of us are very unlikely to ever get psychosis, but this depends on a combination of our genes and the life we live.  In a world in which no-one used cannabis, people would still develop psychosis, sometimes this is a one-off episode of a few days or weeks, sometimes someone dips in and out of periods of psychosis over their lifetime, sometimes it is more or less constant, and occurs with other problems (schizophrenia). All of us have an inbuilt genetic risk for psychosis that ranges from virtually zero up to a maximum of more than 50% (the risk if your identical twin has schizophrenia). Then, on top of that inbuilt risk, things in our lives act as nudges towards or away from psychosis developing.

Someone with a high genetic risk might only need a little nudge, someone with a lower risk may need many nudges all added together for psychosis to emerge. These nudges towards psychosis can include challenging, isolated lives, losing a parent, facing hostility (e.g. the stresses of being a gang-member or a struggling immigrant) stressful events (like having a baby), or even just living in a city. It has become clear that THC in cannabis should be added to this list. Heavy use of high-THC cannabis like skunk seems to nudge people towards developing psychosis, makes it appear earlier in people who might have got it anyway, makes psychosis worse in people living with it already, and makes it harder to recover.

Overstating the role of cannabis in psychosis could be as harmful as ignoring it. For young people who are developing psychosis, a supportive, open, understanding family environment works against psychosis and for recovery, whilst a hostile, blaming response worsens the outcome. People in the process of developing schizophrenia often turn to cannabis, (rather than the cannabis causing the illness) possibly because it feels like it helps them in dealing with some of the symptoms. Understanding their perspective, recognising that cannabis alone is unlikely to have caused the problem and supporting them to live without cannabis (or first to swap skunk for hash) is a more helpful response than angrily blaming them, and considering their life ruined.

Being a cannabis user or being addicted to cannabis seems to go along with problems like depression and anxiety a bit more often than would happen just by chance. It is hard to work out though whether the cannabis causes these problems or if the cannabis use is a result of them, and individuals are using it to self-medicate. It can also be both the drug use and the problems that are caused by something else entirely. Looking at the best evidence on this question, it seems that heavy cannabis use, especially from a young age probably does increase an individual’s chance of becoming depressed or suffering from an anxiety disorder, although the increase is not huge. In fact, more recent studies are finding that CBD, one of the chemicals in cannabis is particularly effective at combatting anxiety disorders but to date there is limited data on its effectiveness as a treatment for depression.

We have written blog post on this topic. Once again, it depends. Scientists have shown that if someone young is addicted to cannabis for a long time, maybe smoking it at least several times a week while whilst they are still at school, they might end up doing quite a bit worse on intelligence tests. On average, cannabis users do less well at school and leave education earlier too.

Chemicals in cannabis might directly interfere with the long slow development of an adult brain, which doesn’t finish until people are in their mid-twenties. Cannabis interferes with memory, so someone might not do well if they try to learn whilst stoned, or even the day after being stoned.

However, cannabis might not be the direct cause of lower achievement, or not the only cause. Researchers looked at twins who differed in their cannabis use but had the same family background, it was the disadvantaged background not the cannabis use that made the difference to their success. Another reason might be that the unfair ‘stoner’ stereotype of lazy, muddle-headed cannabis users (especially young males) subconsciously affects how they start to think about themselves or lowers the expectations other people like teachers have of their abilities, actually leading to lower achievement.

At any age, becoming addicted to a drug makes it harder to succeed in many parts of life. Regularly using drugs such as cannabis can hold someone back simply by using up their time, and re-ordering their priorities, even if the drug itself doesn’t damage the brain.

Some types of cannabis might be more likely to hold young people back. Cannabis with lots of THC and no CBD seems to be more addictive and have worse effects on memory.

To summarise, using lots of cannabis while still young (especially being addicted) could prevent someone reaching their full potential. However, there are many successful and intelligent cannabis users. The same research that found that long-term heavy cannabis use when young might make people less intelligent seemed to show that once people are grown up, starting to smoke cannabis probably doesn’t affect their intelligence. Stereotyping cannabis users as slow thinkers and underachievers can make things worse, because people have an unconscious tendency to be limited by negative stereotypes.

Yes. Cannabis has been used as a medicine for thousands of years and is helping increasing numbers of people now. But some people misunderstand what this means. Sometimes people argue with one side saying “cannabis is a harmful drug” and the other side saying “no it isn’t, cannabis is a wonderful medicine”. Both sides are right and wrong. Cannabis itself isn’t either good or bad; it depends on the type, how it is used, in what situation, and who uses it. It is just like a knife; a surgeon can use one kind of knife to make a sick person better, or a healthy person can accidentally harm themselves with a knife.

Usually, for a drug to be recognized as a proper medicine, scientists have to have done a trial in which they take a group of people who all have a particular problem or illness, give only half of them the drug, and find out if the people who were given the drug do better overall than the people who were not given it. A recent study reported that in the UK nearly 1.4million people are using cannabis for medical reasons. Although cannabis is still illegal it seems that many people are finding a benefit from it. Recently, there has been growing interest in looking at real world accounts of these people and reporting on how medical cannabis has helped them. Scientists conducting this kind of research, which is one of the things we do at Drug Science, argue that this is a reasonable and high-quality way of understanding how well a drug is doing. We are working with legislators to realise the importance of studies that don’t follow strict and expensive trials and they are beginning to see the benefit of this new approach.

Drug Science is currently collecting data on medical cannabis with Project Twenty21, through which eligible people can access medical cannabis at a reduced price, while their treatment outcomes are analysed to gather evidence for the role of cannabis as a medicine and to help guide public health policy.

Some forms of cannabis or chemicals from cannabis have passed through the strict trials showing for example that it can help reduce painful symptoms for some people with MS. Other medical uses of cannabis, like preventing seizures, seem very promising. Other evidence has also emerged on its role in treating chronic pain and anxiety and other psychiatric disorders.

But not everyone can benefit from the medical properties of cannabis, and trying to use it as a medicine without a doctor’s guidance might do more harm than good.

There are many scientists who are trying hard to help patients by investigating and testing all the interesting chemicals in the cannabis plant that could maybe work as medicines, so that as many people as possible can benefit in future. This work takes a long time, especially as research and medical use of the drug is restricted by the law. This leads to a very sad situation where people who might possibly benefit from using a carefully produced cannabis medicine like Sativex cannot get it from their doctor or can only buy unpredictable illegal cannabis.

The distress this causes is made worse when people put their hope in stories, which often spread online, that cannabis is a miracle cure for cancer and almost everything else. These stories are often passed to ill people with good intentions, but they can be a dangerously persuasive mix of misinterpreted facts and total nonsense. They can mislead people into thinking that radiotherapy and chemotherapy are evil treatments that don’t work, their doctors are their enemies, and that all a person with cancer needs is natural cannabis.

There is early hope that cannabis may have an important place in the toolkit doctors have to fight cancer, in short, it may become a useful chemotherapy, but it is not a replacement for current chemotherapy. At the moment, the evidence for cannabis as a chemotherapy is incomplete and uncertain, it might help, it might not. But it is certain that despite the damage they can do, the chemotherapies and radiotherapy in use now have helped millions of people beat cancer or extend their lives.

Cannabis is a Class B drug in the UK. That means that in theory, getting caught by the police with any amount of cannabis can lead to up to 5 years in prison, an unlimited fine or both. However, this political classification of cannabis is not a good guide to its harmfulness compared to other drugs or to the likely legal consequences of getting caught with it.

The first time that the police find cannabis in someone’s possession, they are likely to give a ‘cannabis warning’, and each time afterwards they will take tougher action, such as fining you, or arresting you. It is very unusual in the UK for someone to go to prison just for possessing cannabis for personal use.

Growing cannabis or selling it to others can result in more serious consequences. Remember that the law doesn’t distinguish between dealing cannabis and sharing or giving it to your friends, it all can be counted as supplying the drug.

Getting a criminal record for a cannabis offence can have many bad consequences. It can make it harder to get your dream job or can mean you can be blocked from travelling to some countries such as America. It might make it harder for you to become Prime Minister and produce great laws to minimise the harms of cannabis to society – even though many Prime Ministers and world leaders have admitted to using it!!

Any time you have an opportunity to use cannabis, make sure you are making a thoughtful, deliberate yes or no choice that you can justify to yourself. If deciding not to use the drug doesn’t seem like a realistic option freely available to you, because you are under pressure from other people or if you are perhaps becoming addicted, recognize that your power to limit the risks you face is being threatened and see what you can do to escape that situation.

Remember that your choices go well beyond yes or no. If you are young and think you might like to try cannabis, you don’t necessarily need to say yes at the first opportunity; waiting until you are older reduces the risks, and waiting for a perfect relaxed opportunity rather than the first available chance makes it more likely you’ll enjoy it anyway. If you have chosen ‘yes’ to cannabis use, that firmly closes the door on avoiding risk entirely, but opens up a range of new choices you can make to push those risks down; the hows, whats, whens and whys.

Perhaps consider some or all of these questions:

  1. How am I consuming my cannabis?
  2. Could I switch from smoking to a less harmful method?
  3. Can I avoid mixing it with tobacco?
  4. Would I be satisfied using less, or saving some for another time?
  5. When am I using cannabis?
  6. Is this a risky time and place to get stoned?
  7. Will I need to drive or be responsible for others in the next few hours?
  8. Am I using it more than I used to?
  9. Can I set myself a limit?
  10. Am I taking long regular breaks to confirm I can do without it?
  11. Could I be pregnant?
  12. What cannabis am I using?
  13. Would I prefer something a bit less strong?
  14. Could I find and switch to a lower-risk type of cannabis such as hash?
  15. Why am I using it?
  16. Do I really want this one?
  17. Am I just rolling a joint out of habit?
  18. Could I be becoming addicted?
  19. Could I be equally happy doing something else?
  20. Am I feeling under pressure to fit in?
  21. Am I getting stoned to avoid dealing with something?

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