Chlidren who smoke cannabis are up to five times more likely to progress to harder drugs than those who delay experimenting with marijuana until they are older, a study has found.

Chlidren who smoke cannabis are up to five times more likely to progress to harder drugs than those who delay experimenting with marijuana until they are older, a study has found.

Those who started before the age of 17 were between 2.1 and 5.2 times more likely to develop problems with drinking or drug addiction, according to a study of twins.

The findings, published in the Journal of the American Medical Association, lend weight to the "gateway" hypothesis that suggestsmarijuana use leads young people on to harder drugs. But an editorialsaid this did not mean that all young people who tried marijuana would become heroin or cocaine addicts.

Researchers studied 311 pairs of twins of the same sex, where only one twin had used cannabis before 17. Twins share the same background and family experiences and identical twins share the same genetic risk.

The study was conducted in Australia between 1996 and 2000, when the twins were aged 30 on average. The researchers took into account known risk factors such as early drinking or smoking (of tobacco), divorce, sex abuse or depression, and found they had little effect on the results. There was also no difference between fraternal and identical twins.

The authors said the findings show drug use cannot be solely determined by genetic and/or shared environmental factors. Peer pressure and the social context in which cannabis use occurs may also be factors. "In particular, early access to the use of cannabis may reduce perceived barriers against the use of other illegal drugs and provide access to these drugs," they said.

In a commentary on the findings, Denise Kandel of Columbia University said curbing marijuana use should prevent abuse of other drugs.

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