The dangers of cannabis

Picture of a bag of weed

Cannabis is the most widely used illegal drug in Britain. Find out about the health risks associated with its use.

Scroll down to watch a video about saying no to drugs.

There are several health risks associated with cannabis use, including dependency problems, mental health problems and lung damage.

The effects of cannabis

The effects of cannabis vary:

  • Some people may feel chilled out, relaxed and happy, while others have one puff and feel sick.
  • Others get the giggles and may become talkative.
  • Hunger pangs are common and are known as 'getting the munchies'.
  • Users may become more aware of their senses or feel that time is slowing down. These feelings are due to its hallucinogenic effects.
  • A stronger joint (typically when skunk or sinsemilla is used) may have more powerful effects. Some users may moderate these effects by using less cannabis. Others may find it becomes tempting to binge smoke.

Getting hooked on cannabis

As with other drugs, dependence on cannabis is influenced by a number of factors, including how long you’ve been using it, how much you use and whether you are more prone to becoming dependent. 

You may find you have difficulty stopping regular use, and you may experience psychological and physical withdrawal symptoms when you do stop. Withdrawal symptoms can include cravings for cannabis, irritability, mood changes, appetite disturbance, weight loss, difficulty sleeping and, in some people, sweating, shaking and diarrhoea.

If you've only been using for a short while there should be no problem stopping, but after continued regular use of cannabis, stopping can become more difficult. You’re also at risk of getting addicted to nicotine if you roll your spliffs with tobacco.

Risks associated with cannabis

The following risks are associated with cannabis use:

  • Even hardcore smokers can become anxious, panicky, suspicious or paranoid.
  • Cannabis affects your co-ordination, which is one of the reasons why drug driving, like drink driving, is illegal.
  • Some people think cannabis is harmless because it’s a plant, but it isn’t harmless. Cannabis, like tobacco, has lots of chemical 'nasties', which, with long-term or heavy use, can cause lung disease and possibly cancer. The risk is greater because cannabis is often mixed with tobacco and smoked without a filter. It can also make asthma worse, and cause wheezing in people without asthma.
  • Cannabis itself can affect many different systems in the body, including the heart. It increases the heart rate and can affect blood pressure.
  • If you have a history of mental health problems, taking cannabis is not a good idea. It can cause paranoia in the short term, but in those with a pre-existing psychotic illness, such as schizophrenia, it can contribute to relapse. 
  • If you use cannabis and have a family background of mental illness, such as schizophrenia, you may be at increased risk of developing a psychotic illness.
  • It is reported that frequent use of cannabis can cut a man's sperm count and reduce sperm motility. It can suppress ovulation in women and so may affect fertility.
  • If you’re pregnant, smoking cannabis frequently may increase the risk of the baby being born smaller than expected.
  • Regular, heavy use of cannabis makes it difficult to learn and concentrate. Some people begin to feel tired all the time and can't seem to get motivated.
  • Some users buy strong herbal cannabis (also known as skunk) to get ‘a bigger high’. Unpleasant reactions can be more powerful when you use strong cannabis, and it is possible that using strong cannabis repeatedly could increase the risk of harmful effects such as dependence or developing mental health problems.

Further information

For more information on cannabis and the different forms it comes in, see the cannabis section in the Frank website’s A-Z of drugs.

Getting help if you use cannabis

There are several sources of support if you or someone you know has a problem with drugs. Find out more in Drugs: where to get help.

Saying no to drugs

Would you say no to drugs? Find out what choices teenagers Taz and Mike made, and how their lives were affected.

Media last reviewed: 06/09/2013

Next review due: 06/09/2015


Page last reviewed: 09/10/2012

Next review due: 09/10/2014


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The 6 comments posted are personal views. Any information they give has not been checked and may not be accurate.

Sp0ngeb0b said on 03 December 2013

Firstly, before anyone jumps down my throat, I completely support any and all research into cannibis as a pain killer, treatment for glaucoma and anti-sickness during chemotherapy and any other uses those clever boffins can find. But, and its a big but, it has dangerous psychological side effects like lots of other medications and it is addictive and it can ruin lives, like anything can, illegal drugs, prescription drugs and 'legal highs', or getting drunk and choking in your sleep or sticking your finger in a plug socket with wet hands covered in tin foil. Doctors have argued to the uk government for it to be a lower classification but that's politics and has nothing to do with ethics or what's 'fair'. You argue that it's not as dangerous as heroin? Very true but this web page isn't even remotely comparing it to hardcore drugs or 'demonising' it, it's just providing factual evidence backed by valid research, most of which is common knowledge which makes me wonder, if your all so pro-cannibis, and you know better than doctors, police and politicians, why are you googling it in the first place?
I was here looking for help for my partner because he's addicted and its destroying him trying and failing to quit without help. But then my friend is schizophrenic and uses regularly and heavily and wouldn't dream of giving up, he says it works better than his medication from the doctors. Maybe everyone should stop being so impassioned either way because everyone is different. A knife is not evil only the person stabbing you is.

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Raelik said on 24 March 2013

Lets get a few things straightened out here.
nothing is completely harmless, even the essence of life (water) does harm or even kill in large amounts. So I guess that should be classed as a drug, no?
Oxygen is a drug. Apparently...

Drug driving is bad, yes, it causes deaths and the point is that alcohol (totally legal) causes much more deaths on the road.

Same with regards to mental and physical health, even the medications prescribed by your trusted GP can kill you (or turn you to kill yourself).

I'm not trying to say that cannabis (or drugs) are good. Merely explaining in summary that people should research more for themselves before going at it on a website.

More neutral opinions win the day, give both sides to the story.

Personal belief is that between the two sides of any story is the truth. You just need to find it.

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81Mark said on 26 February 2013

Thousands of people are treated each year by the NHS due to cannabis use, i really dont know where people have got the idea that theyre not. If youve got any friends or family who work in the mental health field they'll tell you about the large number of people suffering from psychosis and although people will say that sufferers were already predisposed of suffering from it which is true theres not many people with the view that cannabis doesnt increase the suffering ten fold. As for saying cannabis doesnt kill anyone thats also false, a large number of fatal traffic accidents due to driving under the influence of cannabis do happen and the even darker side is when people under the influence of cannabis causing a crash because of lapse of concentration can and do wipe out whole families as well as children. If you speak to any police officers they'll also confirm this does happen. Also as stated earlier people who have cannabis caused or exaborated psychosis do commit suicide in the hundreds maybe thousands with the overwhelming victims of cannabis being 12-25. Anybody can do a bit of research themselves by speaking to their local police officer or anyone in the medical field specially psych Doctors who will confirm every point made. Cannabis.................. Far from the harmless weed portrayed by the previous three people.

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Roberto Angelone said on 20 April 2012

Seriously, Cannabis is portrayed here as some great demon haunting this world. Cannabis actually should be given its rightful place as a natural remedy to alleviate pain and wasting for very serious medical conditions. I personally suffer from muscle and joint pain that verges on the chronic. Why I should possibly risk imprisonment if I ever find it a useful remedy in the future simply boggles my mind. In some cases it has also helped some people come off Heroin. Let's begin by calling a spade a spade & dispense of convenient scaremongering tactics please.

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richardvp said on 30 December 2009

it is enormously difficult to make a case for criminalising cannabis and so the NHS has to support government policy by highlighting the dangers, even though they are trivial when compared to our embedded social narcotics such as alcohol and tobacco. At least 11 million people aged 16-59 in this country have smoked cannabis and that suggests about 15 million of the population as a whole. If cannabis was so dangerous the NHS would be treating tens of thousands of people each year and similar numbers would be dying of cannabis-related diseases. this is simply not the case, and cannabis remains a relatively safe drug, although to be treated with caution by children and young adults especially if already suffering from any mental illness. As for what type of cannabis is commonly used in the UK, I am amazed that even this professional website does not know the differnce between the leaf and the flowers. And there you have a fundamental problem, the clear ignorance of those "professionals" who seek to give advice to us ordinary mortals who in fact know better. As for the failure to mention the enormous medical benefits of the plant if used appropriately, this is another sad reflection on the state of our health system which banned its use in 1974 under American government pressure.

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Niallds said on 13 January 2009

Is it not obvious that quoting facts taken out of context on what may happen as a result of use does not alter behaviour?
People's experience is markedly different from what is cited here. Most drug use passes off without problem. I believe crying wolf means that the baby gets throuwn out with the bathwater if you'll pardon my mixed metaphors.
Cannabis does not have to be smoked with tobacco to begin with and clearly a cannabis smoking career results in far less total smoking than a tobacco smoking career. You will be aware that tobacco smoking killed over 100,000 people in the UK last year, cannabis smoking killed no-one. If only one side of the argument is given, people will ignore it.

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