Early marijuana use 'leads to problems'

In a large study of Australian twins, researchers found that those who used marijuana before age 17 were two to five times more likely to use other drugs or to develop alcohol or drug abuse or dependence.

" It's important that we, as parents and as a society, recognise that there is an increased risk "
Professor Andrew Heath

It is thought that many factors influence the likelihood that somebody will develop a drink or drugs problem.

But factors such as genetics and family circumstance are likely to be similar for twins.

So researchers are more able to draw meaningful conclusions when they examine other factors such as early use of soft drugs in the development of later problems with substance abuse.

The researchers focused on 311 sets of same-sex twins. In every case one twin began using marijuana before the age of 17, and the other did not.

Higher rates

By the time these twins were interviewed in their late 20s and early 30s, the early marijuana users had developed higher rates of problems with alcohol and other drugs.

Some 46% reported that they later abused or became dependent upon marijuana, and 43% had become alcohol dependent.

The early marijuana users also used other drugs at higher rates, including cocaine and other stimulants (48%) heroin and other opioids (14%) and hallucinogens (35%).

The rates were between 1.8 and 5.2 times higher than the rates observed in the twins who did not use marijuana before the age of 17.

Researcher Dr Michael Lynskey, of the Queensland Institute of Medical Research in Brisbane, said: "We actually were expecting that by using twins and controlling for genetic and familial effects, we'd find the association between early use and later abuse would disappear.

"But this study demonstrates that there is more to the relationship than we previously thought."

Professor Andrew Heath, of the Missouri Alcoholism Research Center at Washington University School of Medicine, led the study.

He said: "One important thing to say to the parents of a 16-year-old using marijuana is that the majority of kids who use cannabis do not go on to experience problems with drugs or alcohol, but it's important that we, as parents and as a society, recognise that there is an increased risk."

Risk under-estimated

Professor Heath said marijuana use by children and their parents was now so common that society may downplay the risks involved.

The researchers said the reason why early use of marijuana was linked to later problems was not clear.

Professor Lynskey said: "It often is implicitly assumed that the association between cannabis and other drugs is somehow pharmacological, that using cannabis changes your brain or makes you crave other drugs.

"But there are a number of other potential mechanisms, including access to drugs, willingness to break the law and likelihood of engaging in risk-taking behaviours."

The research is published in the Journal of the American Medical Association.