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On the adverse effects of cannabis use

Published 4 April 2006. Updated 1 June 2009.

There are still conflicting views regarding the health hazards of cannabis, as the matter has not been studied sufficiently. It is fairly certain that the use of cannabis can cause immediate health hazards for people experimenting with it or for occasional users. You should also be aware of the risks of long-term use if you are thinking of trying or using cannabis.

Cannabis is the most widely used drug. The intoxicating effects of cannabis vary, depending on how strong the substance is, how much is used, how and where it is used, and the individual characteristics of the user. Cannabis is not toxic. There are no known cases where the use of cannabis alone would have caused death by poisoning.

Cannabis intoxication causes behavioural or psychological changes, such as deterioration of motor functions and a feeling of pleasure; sometimes anxiety, impaired judgement or social withdrawal. Concentration, memory, and attention to detail are impaired. Cannabis reinforces the effect of other intoxicants, makes you less alert and slows down your accuracy and reactive ability. The risk of accidents may increase if you drive while under the influence of cannabis. Those with a hereditary predisposition for mental illnesses may also begin to exhibit signs of serious mental disturbances.

The effects appear during or soon after using cannabis. Typical physical reactions include bloodshot eyes, increased appetite, dryness of the mouth and increased heart rate. When cannabis is smoked, the effects appear within a few minutes. The intoxicating effect lasts for a few hours.        
Regular, long-term use of cannabis is likely to cause irritation and infections in the respiratory passage, as well as malignant tumours, like lung cancer. Some long-term users of cannabis may become addicted, meaning that they can no longer discontinue or control their use. Long-term use also involves deterioration of attention and memory capacity if the substance is used so heavily that the user is intoxicated virtually all the time. During a long period of abstinence the affected attention and memory abilities may improve.
There are also findings to suggest that the use of cannabis may increase the risk of cancer in the mouth, throat and gullet. Cannabis use during pregnancy may be a cause of leukaemia in the child. Cannabis use may impede adolescent success at school and adult performance in work tasks that require good mental capacity.

Even though individual differences are great, it is likely that the health hazards are more pronounced in certain groups. Adolescents who do not do well at school and have started to use cannabis at a young age are more likely to start using other illegal drugs and become addicted to cannabis. Cannabis use during pregnancy may increase the likelihood of premature labour. The use of cannabis may aggravate the symptoms of certain diseases, such as asthma, bronchitis, schizophrenia, and alcohol or other drug addictions.

Pekka Heinälä
Specialist in psychiatry
A-Clinic Foundation 


Hall W, Solowij N. Adverse effects of cannabis. Lancet 1998; 352: 1611–1616.

Heinälä P. Kannabis ja sen terveysvaikutukset (Cannabis and its health effects). Duodecim 1998; 114:2115–2120.