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Marijuana use before age 17 may lead to later drug abuse
According to an article published today in JAMA, a study of over 300 pairs of Australian twins found that those who had used cannabis before the age of 17 had a 2 to 5 times increased risk of drug problems and drug dependence later in life compared to their twin siblings who did not use the drug.
The researchers from the Department of Psychiatry at Washington University School of Medicine, Queensland Institute of Medical Research in Brisbane, Australia, commented to UPI news that they were surprised by the results. “We thought there wouldn't be an increased risk." Michael T. Lynskey, PhD, the lead researcher, said that he was skeptical of the gateway hypothesis, which holds that use of marijuana leads to use of harder drugs such as cocaine and heroin but conceded, "The weight of evidence from our study supports the gateway hypothesis."
The conclusions from the JAMA study point out that “associations between early cannabis use and later drug use and abuse/dependence cannot solely be explained by common predisposing genetic or shared environmental factors. The association may arise from the effects of the peer and social context within which cannabis is used and obtained. In particular, early access to and use of cannabis may reduce perceived barriers against the use of other illegal drugs and provide access to these drugs.”
The study found that more than 40 percent of the twins who were early marijuana users went on to develop problems with abuse or dependency on marijuana or alcohol. Results also showed that less than 32 percent of those who did not have early marijuana use went on to develop later problems with marijuana or alcohol. These early marijuana users were more likely to abuse or become addicted to cocaine, heroin and hallucinogens. Approximately 12 percent had problems with cocaine and heroin and 3 percent had reported problems with opioids. Those who were not early users of marijuana showed 4 percent cocaine and heroin and 1 percent opioids.
The researchers concluded that early exposure to marijuana appeared to be the cause of later drug problems because environmental and genetic factors could not explain the findings. Each twin set was raised together and thus exposed to the same environmental cues. Also, half of the sets were identical twins, meaning they essentially had the same genes. The researchers also controlled for other risk factors for drug problems, such as early alcohol or tobacco use, childhood sexual abuse, depression, social anxiety and parental conflict or separation.
Lynskey pointed out, however, the vast majority of those who used marijuana early in life "did not use other drugs or experience drug-related problems. It was by no means inevitable they would progress to use of other drugs."
Source: JAMA. 2003;289:427-433
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