Cannabis is known for its pain-relieving and mind-altering properties. There are many different strains (species) of the cannabis plant, and all of them have different chemical compositions and effects. Scientists are still identifying new varieties, each with a different chemical make-up, while also making new discoveries about cannabis’ therapeutic potential. Nowadays, cannabis is the most-used illicit drug in the world. While it is safer than other drugs, such as alcohol, cannabis has negative effects on health.

Please read ME for more general information about recreational drugs.


It's not harmless


Drink water




We have listed the most common effects of cannabis. Please note that everyone can be affected differently, and this is not an exhaustive list. Not everyone experiences these effects, and other effects are possible.

The likelihood of feeling these effects, and the strength of the sensation, depends on the THC:CBD ratio of the cannabis.

The effects are (from positive to negative):

    • Feel relaxed
    • Talkative and sociable
    • Happiness
    • Laughter
    • Increased appetite
    • Distorted sense of time and space
    • Alteration of sight and smell
    • Loss of pain perception
    • Impairment of short-term memory
    • Fatigue
    • Paranoia

* Much more common among first-time users

Dose and Onset

How? How much? When? For how long?

Read our section on dosing and tolerance in ME for more information.

The effects you experience from a given dose of cannabis depends on many different details about the user, such as their weight, the speed of their metabolism, their past experiences and the situation in which the cannabis is consumed.

The same is true of every other drug, but with cannabis, it is especially difficult to identify one’s ideal quantity, due to the variety of strains, the way you consume it, and the distribution of the active compounds in the buds.

How you take it matters... From the healthiest to the most harmful way:


This route of administration is the healthiest, but it can only be obtained with a medical prescription in countries where this is legal and sold. As such, it is barely used among recreational drug users, who for the most part are left with more calorific or carcinogenic alternatives. Sativex is the most widely-known spray. It contains high levels of both THC and CBD, and takes around 15 minutes to kick in.


Vaping involves heating up cannabis to release THC and other cannabinoids. Vaping is, crucially, a cleaner alternative to combustion (i.e. smoking/inhaling). The heating process creates a temperature cool enough to avoid creating the toxins that appear during combustion. Therefore, vaping is likely to reduce the coughing and throat irritation associated with smoking. It also doesn’t require any tobacco, and therefore may reduce tobacco addiction in cannabis users3,7,8. This may be the most expensive method of administration, because of the price of the vaporiser. However, vaporising could be a cost-effective method in the long term8,9.

Read about how vaporisation works.


Eating is probably the easiest and safest way to consume cannabis. However, eating cannabis is not without dangers: new users anticipate similar timing and effects to when they are vaping/smoking/inhaling, but the effects take much longer (up to 2 hours) when consuming edibles. Therefore, it’s tempting to re-dose before the high comes on. Be patient: if the first dose wasn't strong enough, try a higher dose another day. Re-dosing is the easiest way to overdose! People normally feel that edibles give them a “body high,” this is because the compounds in cannabis are slightly altered by the stomach, intestine and liver.


Cannabis can be inhaled through a pipe or a bong. While it's less harmful than smoking, it is easier to inhale greater quantities of smoke. Users should be careful not to breathe too deeply in order to regulate their high. Bongs with ice catchers can reduce the irritation of the throat as the ice cools down the smoke.


Smoking cannabis is the most common way to consuming cannabis, even though it’s the unhealthiest. This method increases the probability of developing lung cancer because burning cannabis (with or without tobacco) creates toxins associated with cancer. In Europe, cannabis is often mixed with a much deadlier companion: tobacco. Adding tobacco to your cannabis increases the chances of throat irritation, and also increases the risks of cancer and dependence.

nueroscience info toggle Click the brain for neuro-info!

Written by Chandni Hindocha, PhD student at UCL

How much?


Every time you try a new strain, or use a different supplier, smoke a smaller dose than you are used to, so you can notice variations in the effects and potency of the drug (we define potency as the amount of active ingredient required to produce a desired effect).

  • Light dose: 0.05 g
  • Common dose: 0.15 g
  • Strong dose: 0.25 g

For someone consuming the average dose, a one gram bag of cannabis will last between 8–12 smokes, if you roll your joints with king skins and mix your weed with tobacco.


When you cook edibles, there are even more factors that influence the effects you'll get from the food. Some of these are: the strain you are using, whether you are using oil or butter, the time you leave it cooking, the strength of the heat (low heat, between 1-2 hours in a saucepan is optimum), and of course, your culinary skills...

  • Light dose: 2-4 mg
  • Medium dose: 3-8 mg
  • Strong dose: 4-15 mg

When do the effects kick in and for how long?11,12

The START time below is when you will usually begin to feel the effects of cannabis from the time when you first take it. DURATION is roughly the length of time you will experience the effects, after which the effects will start to wind down and you might start to feel the comedown effects.


  • START: 2-10 minutes
  • DURATION: 2-8 hours


  • START: 20-120 minutes (depends on stomach content)
  • DURATION: 3-9 hours

Kief is the resin that falls off dry cannabis or what accumulates in grinders. It is the strongest part and contains high concentrations of THC. Watch out for this.

How often can I take it?

Consuming cannabis too often decreases the potency of this drug on your body. THis may lead to consuming larger amounts of cannabis. Save cannabis for special occasions or events only, and use it sparingly.


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Click one of the drugs below and see how it mixes with .



Harm reduction

There are certain precautions you should take before using cannabis. The advice below helps to prepare you both physically and mentally. We want you to be safe and enjoy your experience as much as possible, so if you have a bad experience or are struggling with especially bad after-effects, please take note of the advice below.




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Read more about dependence here.

Contrary to what many weed smokers believe, addiction and dependence can occur when using cannabis. Medical practitioners call it Cannabis use disorder. However, it seems to be true that cannabis shows much lower dependency rates than other recreational drugs, such as alcohol and nicotine.

In the long term

Cannabis can have long-term effects when used heavily (i.e. daily or almost daily).

Check and compare your drug use anonymously with the Drugsmeter Cannabis app.

However, all the negative effects of cannabis seem to wear off after a relatively short period of total abstinence, usually 4-6 weeks1, 2.

Cannabis compounds affect a system that is crucial for the development of your brain. Therefore, people who started using cannabis during adolescence may develop:

  • Impaired spatial orientation
  • Impaired verbal fluency
  • Reduced inhibition

In terms of the effects on daily users, there are discrepancies in the scientific literature on the subject. But the most common negative effects are thought to be4, 16:

  • Difficulty making decisions
  • Difficulty learning new things
  • Difficulty making future plans

If you mix cannabis with tobacco, you significantly increase the harms, and aggregate the risks of developing health problems.

Learn more in our section "How you take it matters".

Lung cancer

Interestingly, THC has been found to stop tumours' growth and metastases. In several studies, cannabis smokers (using only pure cannabis) were found to have a reduced risk of lung cancer. Although THC has a beneficial effect on cancer cells, you need to look for balanced levels of THC:CBD, in order to reduce the harm to your brain.

Chronic obstructive pulmonary disease is a broad term that includes different lung diseases. They all result in difficulty of breathing.

Watch this video if you want to know more about medical cannabis and its impact on human health.

When you want to quit...

While withdrawal symptoms are mild and can be easily overcome, weaning yourself off weed does have some unpleasant effects. As with every single drug, the intensity of the withdrawal depends on the quantity you’ve trained your brain and body to tolerate/expect.

Here are some psychological and physical withdrawal symptoms you may experience when you are trying to quit:

  • Craving
  • Insomnia and nightmares
  • Demotivation
  • Lack of enjoyment
  • Aggressiveness and/or anger
  • Irritability
  • Unpleasant corporal symptoms
  • Decreased appetite
  • Nausea
  • Weight loss
  • Mood changes

Remember: Don't hesitate to look for medical help: you won't get in trouble.

Nicotine is often consumed with cannabis. Some withdrawal symptoms are common for most recreational drugs, but for daily users of cannabis and tobacco combined, the double withdrawal is stronger than if you choose to consume cannabis on its own. This is another great reason to go for vaping/eating.

The Law


  • UK: Illegal Class B drug. Possession: up to 5 years in prison, an unlimited fine or both. Supply and production: up to 14 years in prison, an unlimited fine or both.

  • France: Illegal but permitted for medical products.

  • Germany: Not prosecuted up to 6-15 grams depending on the Ländern. Only legal when a permission by 'Federal Institute for Drugs and Medical Services' for medical use is obtained.

  • Netherlands: Legal (less than 5 grams is considered personal use).

  • Norway: Legal (less than 15 grams is considered personal use).

  • Spain and Portugal: Legal (only private growing and smoking are legal).

  • Switzerland: Legal (growing up to four plants is legal).

  • Russia: Illegal, but decriminalised up to 6 grams.


  • USA: Illegal at the federal level, but legal for medical/recreational use in the states of Colorado, Washington, Alaska and Oregon as well as in some cities.

  • Canada: Illegal, with exceptions in some cities.

  • Mexico: Possession is illegal but decriminalized up to 5 grams. Growing and selling are illegal.

Asia and Australasia


Watch this video on why cannabis was criminalised:

More information, references, useful links...


Is cannabis harmless?

Using cannabis is neither harmless nor free of potential health-related problems. However, it appears to be one of the safest drugs and it can be even use for therapy when used correctly.

Does cannabis make you 'dumber?'

Occasional consumption has no effect on any cognitive skill. However, heavy users can experience long-term impairment of some cognitive skills.

Can using cannabis lead to schizophrenia?

There are big discrepancies in the scientific literature, but a clear causal relationship between cannabis use and schizophrenia has not been found2.

What's the difference between Sativa and Indica?

Sativa strains tend to give an uplifting and energetic high while indica strains give a relaxing and calming high. However, this classification seems to be flawed!18

Will smoking cannabis lead me onto using more dangerous drugs?

Cannabis is said to be a 'gateway drug,' suggesting that the use of this drug leads users to consume other illicit drugs such as cocaine and amphetamines. There is actually no evidence to prove the gateway drug theory. There seems to be an association between the consumption of cannabis and other illicit drugs, but we don't know if this relationship is a matter of causation, or simply correlation.


  1. Crean, R. D., Crane, N. A., & Mason, B. J. (2011). An evidence based review of acute and long-term effects of cannabis use on executive cognitive functions. Journal of addiction medicine, 5(1), 1.
  2. Curran, H. V., Freeman, T. P., Mokrysz, C., Lewis, D. A., Morgan, C. J., & Parsons, L. H. (2016). Keep off the grass? Cannabis, cognition and addiction. Nature Reviews Neuroscience, 17(5), 293-306.
  3. Earleywine M, Barnwell SS. Decreased respiratory symptoms in cannabis users who vaporize. Harm Reduct J (2007) 4:11. doi:10.1186/1477-7517-4-11.
  4. Filbey, F. M., McQueeny, T., Kadamangudi, S., Bice, C., & Ketcherside, A. (2015). Combined effects of marijuana and nicotine on memory performance and hippocampal volume. Behavioural brain research, 293, 46-53.
  5. Freeman, TP., Morgan, C.J.A., Hindocha, C., Schafer, G., & Curran, H.V (2014). Just say ‘know’: how do cannabinoid concentrations influence users’ estimates of cannabis potency and the amount they roll in joints? Addiction 109(10):1686-94; doi: 10.1111/add.12634
  6. Hall W, Room R, Bondy S. Comparing the health and psychological risks of alcohol, cannabis, nicotine and opiate use. In: Kalant H, Corrigan W, Hall W, Smart R, eds. The health effects of cannabis. Toronto: Addiction Research Foundation, 1999, pp. 477-508.
  7. Hindocha, C Freeman, T.P., WInstock, A.R, Lynskey, M.T. (2016) Vaping cannabis (marijuana) has the potential to reduce tobacco smoking in cannabis users. Addiction 111(2); 375 – 375; doi: 10.1111/add.13190.
  8. Hindocha, C., Freeman, T.P., Ferris, J.A., Lynskey, M.T., & Winstock, A.R., (2016) No Smoke without tobacco? A global overview of cannabis and tobacco routes of administration and their association with intention to quit. Front Psychiatry, 7, 104.
  9. Hindocha, C., Shaban, N. D., Freeman, T. P., Das, R. K., Gale, G., Schafer, G., ... & Curran, H. V. (2015). Associations between cigarette smoking and cannabis dependence: a longitudinal study of young cannabis users in the United KingdomDrug & Alcohol Dependence148, 165-171.
  14. Iversen, L. L. (2001). The science of marijuana. Oxford University Press.
  15. Kleiber D, Soellner R, Tossmann P. Cannabiskonsum in der Bundesrepublik Deutschland: Entwicklungstendenzen, Konsummuster und Einflußfaktoren. Bonn: Bundesministerium für Gesundheit, 1997.
  16. Kouri EM, Pope HG. Abstinence symptoms during withdrawal from chronic marijuana use. Exp Clin Psychopharmacol 2000;8(4):483-92.
  17. Nutt, D. (2012). Drugs without the hot air. Minimising the Harms of Legal and Illegal Drugs. Cambridge: UIT Cambridge Ltd Nutt, D. J., King, L. A., & Phillips, L. D. (2010).
  18. Piomelli, D., & Russo, E. B. (2016). The cannabis sativa versus cannabis indica debate: an interview with Ethan Russo, MD. Cannabis and Cannabinoid Research, 1(1), 44-46.

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