One in four at risk of Cannabis Psychosis

Study Led By Avshalom Caspi and Terrie Moffitt, of the Institute of Psychiatry at King's College London 2005.

‘ONE in four people carries genes that increases vulnerability to psychotic illnesses if he or she smokes cannabis as a teenager, scientists have found. ‘

A common genetic profile that makes cannabis five times more likely to trigger schizophrenia and similar disorders has been identified, increasing pressure on the UK Government to reverse the drug's reclassification from Class B to Class C.

The increased risk applies to people who inherit variants of a gene named COMT who also smoked cannabis as teenagers. About a quarter of the population have this genetic make-up, and up to 15 per cent of the group are likely to develop psychotic conditions if exposed to the drug early in life.

Neither the drug nor the gene raises the risk of psychosis by itself.

The study, led by Avshalom Caspi and Terrie Moffitt, of the Institute of Psychiatry at King's College London, offers the best explanation yet for the way that cannabis has a devastating psychiatric impact on some users but leaves most unharmed. Scientists had suspected that genetic factors were responsible for this divide, but a gene had not been pinpointed.

The findings, to be published in Biological Psychiatry, also reinforce a growing consensus that nature and nurture are not mutually exclusive forces but combine to affect behaviour and health. The King's team has previously
identified genes that raise the risk of depression or aggression, but only in conjunction with environmental influences.

COMT was chosen as it is known to play a part in the production of dopamine, a brain-signalling chemical that is abnormal in schizophrenia. It comes in two variants, known as valine or methionine, and every person has two copies, one from each parent.

A recent study at Maastricht University found that cannabis doubles the risk of schizophrenia, hallucinations and paranoia among a genetically susceptible group.