Teens who smoke marijuana at an early age are more likely than their siblings to use more dangerous drugs later on or to become addicted to drugs or alcohol, according to a study of twins published Tuesday.


The study found that a twin who smoked marijuana before the age of 17 was up to four times more likely to use other drugs and up to six times more likely to abuse drugs or alcohol, compared to their twin sibling who did not smoke at an early age. Other risk factors for drug abuse, including depression, childhood sexual abuse, or a parent's separation or divorce, did not affect the findings.


It is not clear from the study why using marijuana early on might lead to subsequent drug use and raise the risk of addiction. One possibility is that early and positive experiences with the drug encourage continued use and experimentation with other drugs. People who smoke marijuana might also have access to other drugs, suggest researchers.


Regardless, the study on twins who were exposed to similar environmental and genetic influences, highlights the dangers of early marijuana use, Dr. Michael T. Lynskey, a researcher from Washington University in St. Louis, Missouri, and colleagues, conclude.


The findings also suggest that targeting individuals who begin to smoke marijuana at an early age might be a way to prevent later drug abuse, said Lynskey in an interview with Reuters Health.


"While efforts have focused on preventing the initiation of drug use, there is also a need for efforts to prevent the escalation of use among those who have (used drugs), especially those commencing at an early age," he said.


The study is published in the January 22nd/29th issue of the Journal of the American Medical Association.


The past decade has seen a rise in the number of people entering drug treatment programs for marijuana-related problems. This trend has raised concerns about the long-term consequences, since it is believed that marijuana often precedes the use of "harder" drugs such as cocaine and heroin. This idea, known as the gateway hypothesis, states that there is an order of drug use for most people, beginning with marijuana.


To test this theory, the researchers looked at 311 sets of identical and fraternal twins in Australia who were aged around 30 years, and asked them about their past and present use of drugs and alcohol. In all cases, one twin smoked marijuana before age 17 while the other twin did not.


An analysis of a larger group of 2,765 pairs of twins suggests that identical twins were more likely than fraternal twins to both report smoking marijuana before age 17 years, suggesting that there is a genetic element to trying this drug at a young age, according to the researchers. Earlier use of the drug, as well as early and regular use of cigarettes and alcohol, was also associated with a risk of using other drugs.


"Drug use, and escalation to problem use, is a complex process with multiple determinants, including genetic factors, social factors, family environment, peer influences and so forth," Lynskey said in an interview.


While marijuana use is associated with the use of other drugs, it's not clear if pot actually causes users to go down the road to addiction, according to an editorial by Dr. Denise Kandel, of Columbia University and New York State Psychiatric Institute, New York.


She notes that "progression is not inevitable."


"Not all those who try marijuana will subsequently use cocaine or become heroin addicts," she writes. However, interventions that target marijuana users may help prevent the use of other illicit drugs, according to Kandel.