Cannabis
Cannabis is widely used worldwide with almost 230 million users in 2022 (World Drug Report 2024). It is a drug that comes from the plant Cannabis sativa L and contains hundreds of unique chemical substances including cannabinoids and terpenes. The two main components of cannabis are THC (delta-9-tetrahydrocannabinol), which is psychoactive and causes the "high," and CBD (cannabidiol), which is psychoactively inactive and gives cannabis its calming effects. Cannabis sativa L plants containing very small, non-intoxicating amounts of THC, are also called hemp, are mainly used for textile fiber and for their edible seed oils.
Other names for cannabis include marijuana, weed, skunk or hash.
Products derived from the cannabis plant come in many forms including buds or flower, hash (or hashish), oil, edibles and concentrates (dabs).
Cannabis can be consumed in different ways, by:
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smoking:
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joints (pure cannabis) or spliffs (mixed with tobacco)
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pipes and bongs (water pipes)
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blunts (cigar wrappers filled with cannabis)
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drinking or eating:
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teas
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sodas
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cannabis oil
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tinctures or sprays
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baked goods
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vaporising:
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Vaporising (breathing in dried cannabis through a dry herb vaporizer)
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Vaping (breathing in liquid cannabis vapours through a vaping device/pen)
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dabbing (breathing in very hot vapours from heating cannabis concentrates)
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directly applied to the skin through lotions and balms
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 halfway between vaping /smoking and oral use.
Cannabis is used for recreational as well as medical, social or religious purposes.
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Cannabis affects the body and brain primarily through its active compounds, called cannabinoids. There are more than 100 cannabinoids identified each with their own pharmacology and properties. These compounds interact with the body's endocannabinoid system, which helps regulate many functions like mood, appetite, pain, memory, and sleep. The specific effects depend on the balance of these compounds, the method of consumption, and the individual’s unique body chemistry.
THC is the main psychoactive compound in cannabis, and it's responsible for the "high" people feel. It binds to cannabinoid receptors in the brain, particularly the CB1 receptors, which are involved in mood, memory, and coordination. This interaction can cause feelings of euphoria, altered perception, and impaired coordination. CBD, on the other hand, is non-psychoactive and has calming effects. It interacts with other receptors in the body, like CB2 receptors, which are more involved in immune function and inflammation. CBD may help reduce anxiety, pain, and inflammation without causing a "high."
Read more about the endocannabinoid system here.
Physiologically, cannabis temporarily increases blood pressure and resting heart rate.
Cannabis affects everyone differently, so the experience can vary based on individual factors like amount used, method of consumption, tolerance and the type or strain of cannabis used.
Here are some common effects:
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Euphoria or "high": This is the most well-known effect, caused by THC, which can make users feel happy, relaxed, or even giggly.
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Altered perception: Cannabis can change how users experience time, colours, and sounds, making them seem more intense or pleasurable.
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Increased appetite: Often called the "munchies," cannabis can make you feel very hungry.
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Relaxation or drowsiness: Many people feel calm or sleepy, especially when using strains with higher levels of CBD.
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Impaired coordination and memory: Cannabis can affect motor skills, making tasks like driving or operating machinery dangerous, and it can temporarily impair memory and concentration. More information about driving can be found here.
Possible negative effects:
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Anxiety or paranoia: In some people, especially with higher doses of THC, cannabis can cause feelings of nervousness, paranoia, or fear.
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Visual and auditory hallucinations: Very high doses or potent strains can lead to mild visual or auditory hallucinations.
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Increased heart rate: Cannabis can cause a temporary rise in heart rate and blood pressure, which might be risky for people with heart problems but otherwise harmless and short-lived.
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Dry mouth and red (itchy) eyes: These are common side effects, but they’re harmless. Can easily be avoided by drinking plenty of water and using eye drops.
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Nausea/Dizziness (‘whitey’): feeling faint, dizzy, nauseous, sweaty, or lightheaded, and in some cases, vomiting. This can happen after consuming too much or mixing with alcohol but it passes with time and resting.
Mixing cannabis with other drugs makes the likelihood of a negative experience greater, particularly if mixed with alcohol. As with all drugs, mixing can be dangerous and should be avoided. Please check out the Drug Combinations chart & website.
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The long-term effects of cannabis use can vary depending on factors such as how often it's used, the age of the user, and whether it's consumed in moderation or heavily. Some potential long-term effects may include:
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Memory and learning issues: Frequent use, especially at a young age, may impact cognitive function and memory.
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Risk of dependence: About 1 in 10 users may develop cannabis use disorder (CUD), where they feel the need to keep using cannabis even if it negatively impacts their life.
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Respiratory issues: Smoking cannabis can damage the lungs and lead to chronic coughing or other breathing problems, particularly for tobacco-based joints.
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Cannabinoid hyperemesis syndrome (CHS): rare condition caused by regular (daily) and long-term use of cannabis. The syndrome is characterised by repeated and severe bouts of vomiting. CHS only resolves when a person stops using cannabis completely. Read more here.
Not everyone will experience these effects, and some people may use cannabis regularly without significant long-term consequences. However, the risks increase with heavy, chronic use, particularly if started at a young age.
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Cannabis has been used as medicine for thousands of years, but after the United States began its campaign against it in 1937, it was removed from medical references and pharmacopeias around the world.
There is evidence that cannabis can be effective in treating some forms of pain, and there is emerging evidence that it may have additional therapeutic uses. Research will continue to explore potential therapeutic effects of cannabis to help inform individual and public health decisions, including strategies to minimize potential harms associated with cannabis use.
In the UK, medical cannabis has been legal since 2018 and there are three licensed medicines that can be prescribed for specific conditions (Sativex, Epidyolex and Nabilone). But even at the time of writing (November 2024), medical cannabis on the NHS is extremely difficult to access and is limited to a restricted number of severe and treatment-resistant conditions, such as rare forms of epilepsy and multiple sclerosis. Estimations suggest just a handful of patients in the UK have accessed medical cannabis on the NHS. However, private clinics where patients pay for medical cannabis prescriptions are becoming increasingly popular across the UK to help provide patients with the medicine, they need but are unable to access on the NHS. It is estimated thousands of patients attend these clinics each year. These clinics can provide unlicensed medical cannabis for a variety of health conditions, but patients must pay costly bills for this.
Drug Science is running a large observational study in medical cannabis in the UK called T21. Through T21, eligible patients are able to access medical cannabis at a reduced price, whilst their treatment outcomes are analysed to gather evidence for the effectiveness of cannabis as a medicine and to help guide public health policy. You can read a summary of the data and our published papers from the study here. Our YouTube playlist features videos from patients and clinicians speaking about how and why they joined T21, you can access these here.
Learn more about your rights as a medical cannabis patient here and about self-medicating with cannabis here.
To read your rights as a medical cannabis patient, navigate here.
While cannabis is generally considered less harmful than most other drugs and there have been no reported deaths directly attributed to cannabis use, it’s still important to understand the potential risks.
The harm caused by cannabis use can vary widely depending on factors like the person’s age, how often they use it, the method of consumption, and the potency of the cannabis (specifically its THC and CBD content). For some people, occasional use may have minimal effects, while for others, especially heavy or long-term use, can lead to issues like memory problems, anxiety, or dependence. Cannabis dependence, though less common than with other substances, is still possible and can affect a person’s ability to stop using it even if it starts interfering with their daily life.
The only way to completely eliminate the risks of cannabis is to avoid it altogether. However, for those who choose to use it and accept the potential risks, there are many important decisions that can help minimize harm. Key choices include what type of cannabis to use, when to use it, and how to consume it.
An analysis by Drug Science in 2010 in the UK added up and compared the harm caused by 20 different drugs, and placed cannabis roughly in the middle; it causes more harm to society than several other illegal drugs (mostly because there are many more users- cannabis is the most used illegal drug in the UK) but less harm to its users than tobacco and alcohol, both of which have a higher potential to cause serious illness and death but remain legal. Another study by Drug Science 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, safer and better-informed political choices could make cannabis use safer overall.
Cannabis use can be more dangerous for individuals with certain health conditions. People with psychotic disorders, such as schizophrenia, may experience worsened symptoms, though it has also been suggested that CBD may have antipsychotic effects in patients with psychosis. Those with heart conditions may face increased heart rate and blood pressure risks, while individuals with lung conditions like asthma or COPD may experience respiratory issues. Cannabis use during pregnancy can affect foetal development, and people with liver diseases may struggle to metabolize THC properly. Always consult a healthcare provider before using cannabis if you have any underlying health conditions.
Cannabis can be addictive for some people, though it’s generally considered less addictive than substances like nicotine, alcohol, or opioids. About 9-10% of people who use cannabis will develop cannabis use disorder (CUD), which means they become dependent on the drug and find it difficult to stop using it, even if it’s negatively affecting their lives. The risk of addiction is higher for those who start using cannabis at a young age, use it regularly, or have a history of mental health issues. Symptoms of cannabis addiction can include cravings, spending a lot of time using cannabis or recovering from its effects, and continuing to use it despite knowing it’s causing problems, such as issues with work, school, or relationships. While not everyone who uses cannabis will become addicted, it’s still important to be mindful of how often and how much you use.
Research suggests that behavioural interventions, such as cognitive behavioural therapy (CBT), motivational enhancement therapy (MET), and contingency management, can be effective in treating cannabis use disorder. You can learn more about behavioural treatments for substance use disorders on the National Institute of Mental Health website. While there are currently no FDA-approved medications specifically for treating cannabis use disorder or for medically assisted withdrawal, research into potential treatments is ongoing.
Higher concentrations of THC have been linked to an increased risk of cannabis use developing into cannabis use disorder, as well as other health issues.
A person may experience withdrawal symptoms after stopping or significantly reducing heavy or long-term cannabis use, even if they don’t have cannabis use disorder. One study found that 12.1% of frequent cannabis users go through withdrawal. Symptoms can include anger, irritability, anxiety, restlessness, loss of appetite or weight, depression, insomnia, vivid or unsettling dreams, headaches, sweating, stomach pain, and tremors. Cannabis withdrawal is generally less severe than withdrawal from alcohol (and possibly benzodiazepines). Alcohol withdrawal can cause seizures or even be life-threatening.
In the UK, cannabis is classified as a Class B drug under the Misuse of Drugs Act 1971, meaning it is illegal to possess, supply, or produce it. The law states that if caught with cannabis, you could face up to 5 years in prison, an unlimited fine, or both. However, in practice, first-time offenders are often given a cannabis warning by police, and subsequent offenses may lead to fines, arrest, or more serious penalties. Growing or selling cannabis carries harsher legal consequences, with significant penalties for anyone caught producing or distributing it, including sharing with friends, which is also considered supplying by law. Having a criminal record for cannabis-related offenses can lead to difficulties in securing employment, traveling (especially to countries like the U.S.), and could impact future opportunities, such as holding public office.
However, medical cannabis has been legal in the UK since 2018, if prescribed by a specialist for certain health problems. To be eligible for medical cannabis, you need to have a diagnosed physical or mental health condition that has not been relieved by traditional therapies. Learn more about eligibility for medical cannabis here.
Delta-8 THC is another psychoactive cannabinoid found in cannabis. However, products containing delta-8 THC have not been approved by the FDA for safe use. Consuming large amounts of delta-8 THC can lead to medical emergencies, including breathing problems. There is limited research on the health effects of delta-8 THC and other intoxicating cannabinoids and compounds, such as delta-10 THC, THC-O-acetate, THCV, THCP, HHC, HHC-O-acetate, HHCP, and CBN.
Please visit our page on synthetic cannabinoids, also known as spice.
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The best advice is to start low and take it slow so you can get a feel for the strength of the cannabis.
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If possible, avoid smoking cannabis, as it can harm your lungs. Consider using a vaporizer or making edibles instead.
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It’s best to avoid mixing cannabis with tobacco, as this not only increases the harm to your lungs but also makes cannabis more addictive, since your brain starts to link the two substances.
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When using edibles, keep in mind that the effects take longer to kick in, so be patient and resist the urge to take more. Even experienced users can be caught off guard by the strength of edibles.
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If you start feeling demotivated, anxious, or paranoid after using cannabis, it’s a good idea to stop. These feelings are likely to worsen if you keep using it.
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Never drive after consuming cannabis. Always find a safe alternative to get around.
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Cannabis is a ‘gateway’ drug
Cannabis products, along with alcohol and tobacco, are typically some of the first substances a person will come across in life, and many people who use substances start with these before trying others. However, the idea that cannabis inevitably leads to the use of harder drugs is not supported by evidence. While some people who use cannabis may go on to try other substances, cannabis itself does not directly cause this. Factors like environment, social influences, and personal circumstances play a bigger role in whether someone tries other substances.
Cannabis use causes psychosis
The relationship between cannabis and psychosis is complex and depends on several factors, including genetic predisposition, the amount and potency of cannabis consumed (high-potency cannabis carries a greater risk for psychosis compared to weaker types), early or heavy use and underlying mental health conditions. While cannabis use does not directly cause psychosis in everyone, it can increase the risk of psychotic episodes, especially in vulnerable individuals. For those with a family history of mental illness or existing mental health conditions, it’s advisable to avoid or limit cannabis use.
There's a possibility that cannabis use, especially in large amounts, could increase the likelihood of experiencing psychosis. However, since psychosis is relatively rare, it's difficult to definitively link it to cannabis use. Increased risks of psychosis have also been observed in users of tobacco and alcohol. Learn more here and here.
If you are experiencing psychosis that you think is cannabis induced you can access the Cannabis Clinic for Patients with Psychosis for more support.
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