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At the heart of general practice since 1960

Medical effects of cannabis

Following the reclassification of cannabis

last month, Dr Tim Williams reviews the

evidence of adverse health effects

The cannabis plant has been used by humans as a psychedelic and psychopharmaceutical agent for several thousand years. Relatively unknown in the West until the 19th century, cannabis was introduced to Britain from India. It came into widespread recreational use in the 1960s and its popularity has continued ever since.

How does cannabis affect the brain?

Cannabis contains many different chemical compounds, including more than 60 cannabinoids. The three main cannabinoids responsible for the drug's effects are:

ldelta9-Tetrahydrocannabinol (THC)

 · Cannabinol (CBN)

 · Cannabidiol (CBD)

THC is responsible for most of the psychedelic effects of cannabis, acting upon the naturally occurring CB1 cannabinoid receptor system. This is the brain's only cannabinoid receptor, although a second, CB2, is expressed peripherally, principally in the immune system.

The brain produces endogenous cannabinoids that act as potent agonists at the CB1 receptor ­ these are called anandamides after the Sanskrit word for bliss, ananda. It is not known what physiological roles these endogenous cannabinoids have.

In common with other recreational drugs, including heroin, cocaine, amphetamine and nicotine, THC has been shown to cause release of dopamine in the brain. This can explain some of its reinforcing effects1.

Where does the cannabis sold in the UK come from?

Historically the majority of the cannabis consumed in the UK was grown in Morocco. Recently, however, the market share of UK-grown cannabis has increased dramatically. Seeds and equipment for growing cannabis indoors can now be readily purchased on the internet. The quality of the cannabis that consumers are able to produce themselves has increased with modern cultivation techniques.

Is cannabis becoming more potent?

A popular street name for strong cannabis is skunk. This does not contain any different chemicals than regular cannabis but the quantity of THC per gram is greater. Potency is defined as the concentration of THC in the sample.

The range of cannabis potencies available has increased from 3-6 per cent in the 1960s and 1970s to preparations of up to 20 per cent in modern strains, although the average potency of cannabis samples seized by the police has not risen dramatically. Potency can be increased to 50 per cent by extraction - this is called hashish oil. This form of cannabis remains uncommon.

Does cannabis have any therapeutic uses?

Patient groups have advocated the benefits of cannabis in multiple sclerosis for many years. A large, randomised UK trial of the effects of cannabis on MS spasticity was published recently and the main findings are2:

 · There was no significant effect of cannabis extract or THC on objective measures of spasticity when compared with placebo

 · Cannabis extract and THC reduced patient-reported spasticity

 · Cannabis extract and THC reduced patient-reported pain

 · Objective improvements in mobility were demonstrated.

This long-awaited study has not provided the definitive proof of efficacy that many had hoped for, but it does provide good evidence of the patient experienced benefits in MS.

It has been postulated that cannabis-based compounds also have a role in other disorders involving spasticity and pain, and that they may have a role as neuroprotective agents.

What are the risks of cannabis use?

All recreational drugs affect health and the same categories can be used to compare the risks of a range of drugs. Acute medical and psychological consequences can be contrasted with risks to long-term physical and mental health, as well as the harm drug use can cause to society.

Acute medical risks

These are the risks due to the direct effects of cannabis on the body after use. Cannabis causes dilatation of some blood vessels and constriction of others ­ the characteristic redness of the eye is due to dilatation of conjunctival blood vessels. It also causes an increase in heart rate and alters blood pressure unpredictably. These effects are mild and tolerance develops quickly.

Unlike sedative intoxicants such as alcohol, cannabis does not cause respiratory depression or suppress the gag reflex, even during extreme intoxication.

Acute mental health risks

Acutely cannabis can lead to heightened anxiety, panic attacks, confusion and paranoia. These effects are usually shortlived but in some cases can lead to a psychotic state that requires treatment with antipsychotics. Cannabis can precipitate relapse in schizophrenia and presentation in people with no history of mental illness3.

Dependence potential

Cannabis dependence was once contested but is now established to be a real phenomenon. Some

10 per cent of people attending drug treatment services report cannabis as their main drug of dependence. Abrupt cessation of regular use is associated with a withdrawal syndrome. Symptoms include decreased appetite, weight loss, sleep disruption, increased irritability, restlessness, anger.

These symptoms appear to be similar in type and magnitude to those of nicotine withdrawa · 4.

Chronic medical risks

Cannabis users suffer from the health consequences of tobacco smoking. Moreover, cannabis has a higher concentration of certain carcinogens than nicotine.

However, these risks are mitigated somewhat as cannabis users on average smoke fewer cigarettes a day than tobacco smokers and most give up the drug in their 30s, so limiting the long-term exposure which we know to be the critical factor in cigarette-induced lung cancer. There is no evidence to show that cannabis causes any structural brain changes in humans.

Chronic mental health risks

The main concern is whether chronic use of cannabis can lead to schizophrenia. Population studies have reported an increased use of cannabis in people who later develop schizophrenia and this risk appears to increase with the amount of cannabis used. Cannabis use confers an overall twofold increase in the relative risk for later schizophrenia5 and it is therefore prudent to discourage cannabis use in vulnerable populations.

Dangers of intoxication

Cannabis impairs the performance of complex tasks that require sustained attention and motor control, such as driving. Cannabis differs from alcohol in that it does not increase risk-taking behaviour, which may explain its small role in road traffic accidents. Intoxication with cannabis produces relaxation and social withdrawal, in sharp contrast to the aggressive and disinhibited behaviour with alcohol, and therefore scores low on this risk factor.

Cannabis and pregnancy

There is some evidence that cannabis can cause lower birthweights, spontaneous abortion, increased risk of minor birth defects, increased risk of sudden infant death syndrome.

These risks are similar to those found in tobacco-smoking mothers and it is likely that cannabis is as dangerous to the fetus as alcohol or smoking. Reduction and elimination of cannabis use during pregnancy should be strongly advised.

Does cannabis use lead to other drug use?

The theory that cannabis use leads people to take drugs such as heroin and crack is much debated. Studies have shown that cannabis use does have some predictive value for future heroin use. However, it should be noted that early alcohol use and cigarette smoking also predicts future cannabis use.

The driving factor in such associations appears to be the underlying personality. A person who is more likely to experiment with novel experiences will continue to do so given the appropriate environment and opportunities.


Cannabis is much less harmful than heroin and crack but it presents real dangers for those with psychotic vulnerability. The recent change in its legal status reflects this evidence. The evidence of efficacy as a medical treatment is still limited.

Tim Williams is honorary clinical lecturer at the Psychopharmacology Unit, University of Bristol

Status in law

 · Cannabinol and cannabinol derivatives (previously class A drugs) and cannabis and cannabis resin (previously class B drugs) were reclassified as class C drugs on January 29

 · The class of a drug refers to its classification under the Misuse of Drugs Act (1971)

 · All cannabis derivatives remain schedule 1 drugs which are not used medicinally; possession of schedule 1 drugs is strictly controlled

 · The schedule of a drug refers to its classification under the Medicines Act (1968)

 · The reclassification of cannabis means penalties for possession will be reduced and there will be a presumption against arrest of adults for possession

Cannabis use in England and

Wales during 2001/2

Age No. of users

16-24 27 per cent

25-34 13 per cent

35-59 4 per cent

16-59 11 per cent

Source: British Crime Survey 2001/2


1 Ashton C H. Pharmacology and the effects of cannabis:

a brief review.

Br J Psych 2001;178:101-06

2 Zajicek J et al. Cannabinoids for treatment of spasticity and other symptoms related to multiple sclerosis (CAMS study): multicentre randomised

placebo-controlled trial.

Lancet 2003;362(9395):1517-26

3 Johns A. Psychiatric effects of cannabis. Br J Psych 2001;178:116-22

4 Budney AJ et al. Marijuana abstinence effects in marijuana smokers maintained in their home environment. Arch Gen Psychiatry 2001;58(10):917-24

5 Arseneault L et al. Causal association between cannabis and psychosis: examination of the evidence. Br J Psychiatry 2004;184(2):110-117

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