Drug information about Cannabis (ISCD)

Cannabis / Marijuana




Cannabis is a psychoactive plant from south and central Asia that has been eaten and smoked for millennia. More than 3 million people in Europe are thought to use it daily or near-daily, whilst a much larger number are occasional users.

Cannabis resin and plant parts, (particularly the unfertilised female flower ‘buds’) contain delta-9-tetrahydrocannabinol (THC), cannabidiol (CBD), and other related drug chemicals.

Herbal cannabis

This is pieces of dried cannabis plants (usually green). It may be mostly the buds, or may contain leaves, stems or seeds. It varies in strength.


When people use the word ‘skunk’ they usually mean a type of strong herbal cannabis. It is the dried buds of cannabis plants (usually green) that have been selectively bred for high THC content and intensively grown indoors. Skunk is generally strong stuff, but contains little or no CBD. This may increase the risk of harm it poses, or at least the risk of unpleasantly, unexpectedly overwhelming experiences. Users may be able to adjust the amount they take to the strength, which may help moderate any additional risk.

Hash / cannabis resin

This is a substance collected from cannabis plants, which usually looks golden or brown, and can be sticky like soft wax, or more biscuity. It is similar in effects to herbal cannabis, although it is easier to add rubbish into hash to increase weight and profits, which could potentially pose a health risk or make it more unpleasant. Hash can be strong, but it usually contains CBD, so it may cause fewer problems than skunk.

Cannabis oil / hash oil 

A potent, dark, honey-like extract, which can be more than 80% pure THC. It can be dabbed onto cigarettes or joints, vaporised or swallowed. As with skunk, this high potency may increase the risk of health harms, or at least the risk of overdoing it and having a very nasty couple of hours.

Synthetic cannabis products, e.g. ‘Spice’ smoking blends/incense, and cannabinoid ‘research chemicals’

These are not actually a type of cannabis at all, and the active effects are not derived from ‘herbal’ ingredients as may be claimed. A range of cannabinoid chemicals related to the active substances in cannabis are made in laboratories. They have been sold as pure chemicals, (with high risk of overdose) but more commonly are added to smoke-able plant material. Whereas cannabis has a long history of human use and scientific research, the potential harms of these new cannabinoids are almost totally unknown. They are likely to pose greater risks or harm and of having an unpleasant experience compared to cannabis. Cases of agitation, unwanted hallucinations, and seizures have been associated with them. The claims of manufacturers and sellers about what is in these products, and how much, cannot be relied upon. Products which claim to be legal may not be.

What are the different ways people use cannabis?

Smoking it

Cannabis is commonly smoked in one of various ways. Cannabis cigarettes (called joints or spliffs amongst dozens of slang terms) can be made with or without tobacco. Using tobacco in a cannabis cigarette puts the user at risk of all the serious harms that smoking tobacco causes in addition to the risks of cannabis alone. The highly addictive properties of nicotine in the tobacco may also encourage the user to smoke more, increasing the amounts of both drugs used. Cannabis can also be smoked using a pipe, or a water-pipe.

Cannabis smoke is irritating to the lungs, causing coughing as an immediate reaction to inhalation (particularly in new users). Regular heavy use can cause wheezing and coughing. For information on the long term harms of cannabis smoke, see the ‘What are the long-term impacts of of cannabis to health and wellbeing?’ section below.

Using a vaporiser

Vaporisers provide a direct alternative to smoking cannabis whilst reducing exposure to potentially harmful smoke. There are several types, but they all heat the cannabis enough that the drug chemicals evaporate and can be inhaled, without heating it enough that it burns. The vapour can still be irritating and can cause coughing.

Eating/drinking it

The drug chemicals in cannabis dissolve in fat, so some users prepare food containing butter or oil, such as brownies, with the drug. Cannabis prepared with milk and butter (‘bhang’), in a tea or other drink, is a traditional method of consumption in the Indian subcontinent. Whilst this eliminates the harms of inhaling smoke, caution with dose is essential. Eating cannabis is more associated with accidental over-consumption, as the effects don’t come on as fast as they do with smoking, so a user can eat lots without realising the potency, and then find it is kicking in far too strongly an hour or so later. The effects last much longer too: up to 8 hours or more.

There have been cases of the unintended consumption of cannabis-laced food when it has been left around. This can be both dangerous and highly distressing. Children and even pets have needed treatment after this.

The drug chemicals also dissolve into alcohol. ‘Green Dragon’ is one name for spirits which has had cannabis soaked in it. Combining more than a little alcohol with cannabis increases the chance of nausea, vomiting and unpleasant disorientation. Vegetable glycerine has been used as an alcohol-free alternative.

The major psychoactive effects of cannabis are caused by THC. THC stimulates cannabinoid receptors in the brain. The brain contains naturally occurring cannabinoids (endocannabinoids) and THC acts at the same place that is meant for these chemicals. CBD is another cannabinoid in cannabis, and it seems to interact with THC, perhaps dampening down some of the more negative effects of THC. The brain’s cannabinoid system is involved in mood, pain-relief and memory, but the precise details of how cannabis affects it are unknown.

The effects of cannabis are highly variable depending on the strength, method of intake, and many other variables. The same stuff taken in the same way by two different people can give two totally different experiences. Everyone reacts to cannabis differently, for example some people may respond in a way which they enjoy and some people may dislike the effects of cannabis. Some of the effects of cannabis can be unpleasant, and even overwhelming, especially in inexperienced users or with large doses. It is common to feel tense and anxious when using cannabis, or to feel loss of control over thoughts, which can boost anxiety.

Cannabis can make the user feel comfortably drowsy and pleasantly confused, and at higher doses, sedated. The world might seem to spin, and coordination is affected, a bit like being drunk. Usually, users don’t feel like moving much. This is called being ‘stoned’.

However, cannabis can also make the user feel ‘high’, or euphoric, and make the place they are and the people they are with seem special. It changes perception and awareness, so appreciation of music, flavours, sights and humour can be enhanced, whilst time can seem to stretch and distort.

It also changes consciousness in ways that allows unusual abstract thoughts, ideas or memories to float into the user’s mind. It can make people feel that they are reaching levels of understanding and connection beyond what can normally be experienced, which can feel enlightening (which may sound like nonsense to anyone around who’s sober). Some people can find cannabis hallucinogenic to an extent, seeing things changing in size for example. Some people hear voices and sounds. Hearing voices, especially if you have paranoid ideas about them, is likely to suggest that you are at higher risk of serious problems, such as triggering underlying psychosis, as a result of using cannabis.

Physical effects include a raised heart-rate, sometimes heart palpitations, a dry mouth and often red eyes. After effects have peaked, users often have a strong appetite for food, especially sweet and fatty stuff.

Cannabis has a long history of therapeutic use. Medical research has given scientific credibility to parts of this tradition of cannabis as a medicine, and it is available on prescription in some European countries. Other countries have licensed a standardised cannabis mouth-spray (Sativex). Some individuals who suffer from pain and spasticity as symptoms of multiple sclerosis are helped by cannabis. The drug helps some with neuropathic or chronic pain, and also can aid sleep.

Capsules of a synthetic cannabinoid related to THC called Nabilone are also licensed for medical use in some countries. It can be useful to decrease nausea and vomiting caused by AIDS, cancer, and the drugs used to treat them. Cannabis also increases appetite, which can counter wasting.

Research avenues are active into potential effects of cannabinoids on other disorders which could result in future treatments. For example, research suggests that CBD seems to reduce symptoms of schizophrenia (whilst THC, and cannabis as a whole, seems to exacerbate them).

The medical use of marijuana remains highly controversial.

Some people self-medicate with cannabis, using it to manage symptoms and improve subjective wellbeing without medical guidance. The same health risks apply to cannabis whether it is used for pleasure or as a medication. The benefits of using cannabis as a medicine (over alternatives) may often be outweighed by the harms.

Whether they have a doctor’s approval or not, many people who use cannabis medicinally seek to minimise the harms they face, for example by eating or vaporising it instead of smoking.

When smoking or eating cannabis in the most common ways, there is no possibility of dangerously overdosing, but it is easy to take too much and suffer horrible effects, especially when eating it. You may feel sick, confused, frightened, overwhelmed, dizzy and delirious. Inhaling smoke can also be very uncomfortable and irritating particularly if you’re not used to smoking and therefore it may induce coughing fits.

Below are some specific risks that may occur as a result of using cannabis, including the common and not-too-serious and the rare and very serious.


Cannabis increases the chance of accidents. Driving or doing anything that requires co-ordination would be very dangerous.

A ‘whitey’

Unpleasant side effects become more likely with large amounts, particularly of skunk, or when cannabis is consumed too fast. Cannabis lowers blood pressure and this can cause a horrible rush of dizziness, sickness and fainting. These effects are colloquially known as ‘throwing a whitey’ as the person suddenly becomes pale.  If you feel ill, don’t have any more cannabis, have a lie down with your feet up, and try and eat something like a chocolate bar.

Freaking out

Cannabis sometimes causes horrible attacks of powerful anxiety and dread, especially in inexperienced users and those who take large amounts. Skunk may be more likely to cause freak-outs as it usually has little or no CBD. It is thought that CBD reduces anxiety and can reduce some of the negative effects of THC. Cannabis can be quite trippy, especially at very high doses, and especially in certain individuals. It can produce sensations of distorted reality and dissolving identity, and occasionally hallucinations. People describe having unstoppable streams of consciousness and loops of thought running through their head, and a feeling that they have lost control over their mind. The effects of panic, for example hyperventilating and having a pounding heart, can further scare the sufferer and make things worse. The way people react to cannabis is partly down to individual differences and genetics, so if you react to cannabis in this way, it is likely that it will happen again should you continue to use the drug.

If someone starts feeling panicky, the worst thing you can do is sound loud and panicked too. Try to hold eye-contact, maybe hold their hand if you know them, and be calm and soothing, reminding them that the effects will soon fade. Get them to concentrate on taking long….slow…. breaths. Lying down with raised feet, and breathing in and out of a paper bag can help with dizziness. In the very rare event that they do not improve in a few minutes, or if their behaviour could be a risk to themselves or others, you might need to get medical help to prevent injuries or a drawn out traumatic experience.

Triggering the appearance or relapse of mental illness

Any person who already has schizophrenia, has ever suffered psychoses, or has family members who do will be at especially high risk of triggering these problems if they smoke cannabis.  It has long been recognised that in anyone, cannabis (or more specifically THC) can cause immediate effects  similar to psychotic symptoms, but with people with these risk factors, psychotic symptoms could last beyond the few hours when the drug is having its immediate effects. The risk is increased if the type of cannabis contains a high proportion of THC in relation to the other important component of cannabis, cannabidiol (CBD), which may counteract the psychoactive effects of THC. Skunk, which has high levels of THC but lacks CBD, may therefore be more  risky, though there is no direct evidence proving this.

Although it is well-established that cannabis can cause passing psychosis-like symptoms in normally healthy individuals and trigger the appearance of symptoms in patients with or at risk of schizophrenia , the idea that it can cause schizophrenia to develop in healthy individuals is controversial and the evidence is mixed. Whilst over recent decades the use of strong, skunk cannabis has risen, diagnoses of psychotic illnesses have been stable or declined.  What seems more likely is that cannabis can promote the appearance of psychotic illness in individuals that are already susceptible to this.

Yes, if you have schizophrenia or another serious psychiatric condition, or if any close relatives do, cannabis, especially high THC varieties, may trigger serious symptoms. Anyone who suffers from anxiety or panic may find that cannabis can bring on an attack.

People with heart or blood-pressure problems could face increased risks from cannabis.

Asthmatic people may find that the smoke irritates their lungs.

Combining cannabis with other drugs is more likely to cause unwanted unpleasant effects rather than dangerous effects.  If you use cannabis with alcohol you are more likely to feel sick and dizzy and suffer other negative side-effects that the drugs can cause. Combining cannabis and cocaine may increase the likelihood of agitation and paranoia that comes with both drugs. As is true for all drugs, mixing substances makes their effects more unpredictable.

Cannabis can be addictive. Some users find that they gradually need more to feel good, that they feel bad if they don’t use it, and that it is difficult to stop using it, even when it is getting in the way of them living the life they want to live. Studies suggest that around 1 in 10 people who try cannabis become dependent on it at some time and about half of people who use it every day are dependent.

Cannabis may lift mood and relieve stress, which can make it tempting to use it to deal with the tensions of life. However, using any drug as a short-cut to unwinding and feeling good after a stressful day makes it more likely that you will come to feel reliant on the drug, and become dependent. If you begin habitually using drugs or become dependent at a younger age, it may be more likely that this habit will stick in the long-run, as the lifestyle habits you settle in to, or define as normal for you can often be set in youth.

Although most cannabis users do not become dependent, for those that do it can be a big problem. Quitting the drug can seem almost impossible; about the same amount succeed as those trying to quit tobacco. Whilst for a lucky section of users, giving up even a daily habit is not a big deal, others can suffer from a big range of withdrawal symptoms in the first days and weeks after giving up. Aside from cravings which can last months, many feel restless in the first few days, struggle to sleep, have weird dreams and get bad mood swings. Some people quitting have reported an improvement in their memory.

The chance of adults suffering harm from occasional cannabis use is small. Very heavy use, use with tobacco and addiction more commonly causes problems. There could be increased risks too from smoking at a young age, when important brain development is underway.

Physical health effects

Smoking the drug regularly can be bad for your lungs and breathing. Cannabis smoking may increase the risk of lung cancer, although not every study has found this. Further research is needed. Heavy cannabis use can cause wheezing, coughing, breathlessness with exercise and other symptoms. When combined with tobacco, cannabis seems to increase the damage that tobacco can do alone, causing more severe symptoms.

THC seems to impair the immune system, so heavy users could be more vulnerable to illnesses.

Whilst cannabis usually works to reduce vomiting, there have reports of regular cannabis users who suffer episodes of recurrent vomiting, (cyclical vomiting syndrome) which is relieved by hot-water baths or showers. There is not yet much research to confirm the reality and measure the frequency of this odd ‘cannabinoid hyperemesis syndrome’.

Mental health effects

Some evidence suggests that cannabis users, especially heavy, long-term users and people who use it regularly during adolescence are more likely to suffer depression, or anxiety disorders. It is difficult to be absolutely sure that it is the cannabis that actually causes these problems, but the possibility should be taken seriously.

Very heavy, long-term cannabis use seems to cause some negative effects on memory, which may reverse after stopping using the drug.

Evidence has recently emerged suggesting that people who become heavy cannabis users before the age of 18 show a significant decline in their IQ scores. Ending the dependency seems not to fully reverse the loss of IQ points, although it is likely to prevent further decline at least. More research is needed to provide supporting evidence that cannabis use in youth can be damaging to intellectual and cognitive abilities and fill in further detail. Heavy use of most drugs during education and brain development is likely to be damaging. There is no evidence that cannabis causes negative effects on IQ when the drug is taken up in adulthood.

Taking cannabis always has risks, but the risks are not equal to all people who use it. Someone who smokes it heavily on a daily basis from their teenage years is more likely to notice negative effects on their life and health, whereas occasional users are much less likely to, as long as they can avoid becoming regular users. Decisions to avoid cannabis, to use smaller quantities, to use it less frequently and to use it in different ways can all reduce the risks of harm you face.

Could you avoid it? Could you use less?

For some people it may be particularly beneficial to avoid cannabis or minimise use. People who start smoking in their teens may be more likely to suffer harms, and more likely to establish lasting habits of cannabis use. People who have ever had a psychotic episode or schizophrenia, or with close relatives who have, may be at greater risk of worsening their mental health.

Anyone who has suffered panic attacks or anxiety, or is concerned about keeping in control of their thoughts and feelings might be more likely to find the effects of cannabis distressing.

The drug affects everyone differently. If you find that cannabis brings on strong hallucinatory distortions of reality (for example making you hear voices), become paranoid or have delusional thoughts, you are probably one of the people at higher risk from cannabis’ very unpleasant or harmful effects. If you have a horrible experience using cannabis, the chances are high that this will happen again, or get worse in future if you carry on using.

What are you using, how are you using it?

Some methods of taking cannabis are more harmful than others. In the UK, it is normal for cannabis users to mix skunk or hash into a roll-up tobacco cigarette. Smoking these puts the smoker at risk of nicotine addiction, and all the other very serious harms to the lungs and heart associated with tobacco. Smoking cannabis without tobacco is probably much less damaging, though not harmless to the lungs. Using a vaporiser avoids the potential harm of smoke inhalation, but does not take away all the risks. Eating cannabis foods, or using other ways of taking it orally, also avoid smoking harms, but greater care is needed to prevent an overly strong experience, especially as it can take an hour or more for the effects to become fully apparent.

Evidence is emerging that types of cannabis, (such as the herbal cannabis commonly called ‘skunk’) which are strong but contain little or no CBD, may be more likely than other types of cannabis to be associated with harmful and unpleasant effects on mental health. If you are able to choose what you use, this choice could be an opportunity to minimise the risks.

Are you proactively minimising your risk of addiction?

It is a priority for current users wishing to minimise their risk of harms to avoid becoming dependent on cannabis. However responsible your intentions, there is always a risk of this, and some people seem to be predisposed genetically to becoming dependent. Increasing tolerance to cannabis (having to smoke more and more to get the desired effects), is considered a signal of a growing risk of becoming reliant on the drug. Observing your patterns of use can help prevent you only realising a problem has developed when it has become hard to take control again. Being deliberately mindful of your use could help avoid getting to the point where you find that you are rolling a joint out of habit without having really made a decision. Some regular users try to manage the risk of dependence creeping up on them by taking ‘T-breaks’ (the ‘T’ standing for tolerance), where they abstain for a few weeks to keep their use in check and to monitor the extent to which they do or do not crave the drug. Doing this may make it less likely that your habit will slip out of control.

Should you hold the cannabis smoke in for as long as possible?

It is common for cannabis users to inhale from a joint, and then hold the smoke in for as long as they can, with the intention of getting greater effects. The evidence suggests that this increases the potential for smoking harms without doing much to increase the amount of THC getting into the blood. THC is absorbed fast, and after the first few seconds, holding the smoke doesn’t result in significantly higher amounts being absorbed.  However, holding the smoke in for any extended period may cause increased exposure to harmful substances, such as the carcinogens contained in smoke. Long breath-holding also causes minor effects on the heart, including skipped beats, which combined with the effects of cannabis on the heart may increase the chance of unpleasant palpitations and anxiety. The dizziness caused by excessive breath holding may account for the perception that breath-holding works to increase intoxication.


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