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Marijuana's link to hard drug use not genetic

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Marijuana's link to hard drug use not genetic


21:00 21 January 03


NewScientist.com news service


The reason why young cannabis users are much more likely to progress to harder drugs has provoked fierce debate - but a new study of twins has ruled out a strong genetic component.


Researchers looked at over 300 pairs of same sex twins, both identical and non-identical, in which one twin started using cannabis before his or her 17th birthday and the other did not.


Michael Lynskey, at Washington University School of Medicine in St Louis, Missouri, and his team found that the early user was two to five times more likely to go on to use harder drugs or become dependent on alcohol - regardless of whether they were an identical twin or not.


The fact that identical twins, who share all their genes, did not differ from non-identical twins, who share half, suggests that the progression is not the product of genes.



Gateway drug


Many hard drug users have followed a similar path from cigarettes

and alcohol, to cannabis, to heroin and cocaine. This has led some researchers to argue that soft drugs provide a "gateway" to harder substances. Clearly, not everyone who likes a drink ends up as a coke addict, but very few users of hard drugs have not tried cannabis first.


It might be that cannabis users have a genetic profile that predisposes them both to cannabis use and to harder drugs, or a personal history that does the same. But, until now, no one has been able to disentangle the effect of nature and nurture.


The twin study rules out a large genetic component. But it also suggests that the home and womb environment may not a key factor either, because the twins shared both and yet early marijuana use in one still raised the chances of later hard drug use.


However, Lynskey acknowledges that it is impossible to eliminate all nurture differences between twins. For example, one twin might have suffered a traumatic event in childhood that did not affect the other.



Drug policies


Nonetheless, the research shows that the gateway is real, so what

implications does this have for drug laws? "This finding can be used as the bases for quite opposite drug policies," Lynskey told New Scientist.


The problem is that researchers are no nearer understanding the mechanism of the gateway. If taking cannabis causes physiological effects in the brain that predispose the smoker to other drugs, then the message for lawmakers is that keeping people away from cannabis will also keep them off heroin and cocaine.


On the other hand, if cannabis smokers move on to harder drugs simply because they have access to them through drug dealers, then decriminalisation ought to reduce hard drug use.


The work shows that marijuana is indeed a gateway to other illicit drugs, says Denise Kandel, at Columbia University, New York, in an editorial commenting on the journal paper. Therefore, whatever the appropriate intervention turns out to be, it must be focussed on young cannabis users, she says.


Journal reference: Journal of the American Medical Association (vol 289, p 427, 482)


James Randerson

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