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.
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 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, and there has not been enough research done to show for sure that is causes far less harm. Even so, 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 risks to the lungs, but a possible downside is that the effects can take ages 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.