A new study of 311 sets of same-sex twins in which only one twin had smoked marijuana before age 17 has bolstered the fiercely debated "gateway theory" that pot can lead to harder drugs. The study included 136 sets of identical twins who shared the same genetic makeup and all the participants were 30 years of age, on an average, when they were asked about their teenage drug use. The researchers found that those who were early marijuana smokers were five times more likely than their twins to move on to harder drugs. They were about twice as likely to use opiates, which include heroin, and five times more likely to use hallucinogens, which include LSD. They found that about 46 percent of the early marijuana users reported that they later abused or became dependent on marijuana, and 43 percent had become dependent on alcohol. The study also reported that among 48 per cent of the early marijuana users, the most commonly used harder drugs were cocaine and other stimulants as compared to 26 percent of the non-early marijuana users. Hallucinogens were the second most common, used by 35 percent of the early marijuana twins versus 18 percent of the others.
The study, whose lead researcher was Michael Lynskey, was reported in the Journal of the American Medical Association. Lynskey and other colleagues agreed that the study had several limitations like the fact that even identical twins do not share the same environment during adolescence and the study had relied on participants' reporting of their own experiences. Lynskey also added that though it is implied often that using marijuana changes one's brain or makes one crave for other drugs, the study did not show how marijuana could lead to harder drugs. He also said that there are a number of other potential mechanisms, including access to drugs, willingness to break the law and likelihood of engaging in risk-taking behavior that might lure one to harder drugs.