Young people who are genetically vulnerable to depression should be extra careful about using cannabis: smoking cannabis leads to an increased risk of developing depressive symptoms. This has emerged from research carried out by Roy Otten at the Behavioural Science Institute of Radboud University Nijmegen that is published in the online version of the scientific journal Addiction Biology. Two-thirds of the population have the gene variant that makes one sensitive to depression.

Many young people in the Netherlands use cannabis. Nearly 30% of 16-year-olds indicate that they have used cannabis on at least one occasion, and 12% that they have used it during the past month. Besides worse performances at school, the use of cannabis also increases the risk of developing schizophrenia and psychosis. Smoking hashish and weed were thought to increase the risk of depression but no conclusive evidence for this was available to date. Otten suspects that this is partly because his predecessors failed to consider the individual genetic vulnerability to depression.

Long-term study Over a five-year period, data were collected from 428 families and their two adolescent children. Each year the children answered questions on topics such as their behaviour and depressive symptoms. The variant of the serontonin gene (5-HTT) responsible for increased vulnerability to developing depression was also determined. In young people with a special variant of the gene, cannabis use led to an increase of depressive symptoms.

Robust effect 'The effect is robust. It still remains, even if you take into account a series of other variables that could cause the effect, such as smoking behaviour, alcohol use, upbringing, personality and socio- economic status. Some people might think that young people with a disposition for depression would start smoking cannabis as a form of self-medication, and that the presence of depressive symptoms is therefore the cause of cannabis use. However, in the longer term that is definitely not the case. Although the immediate effect of cannabis may be pleasant and cause a feeling of euphoria, in the longer term we observe that cannabis use leads to an increase in depressive symptoms in young people with this specific genotype.'

Knowing what the negative effects of cannabis use could be is important because although cannabis may cause an immediate euphoric feeling, for a large group in the population its use can lead to an increase of depressive symptoms in the longer term.