Weeding Out the Genetics of Marijuana Use
Do Genetics Influence Future Drug Use?

By Ellen Kuwana
Neuroscience for Kids Staff Writer
March 25, 2003

Whether or not marijuana use by teens leads to other drug use has been debated for years. New research from Australia sheds light on this debate with a study of 311 sets of twins, including 136 sets of identical twins. Twin studies are particularly useful to researchers because they provide information on the genetics underlying behavior. This study rules out a strong genetic influence linking early marijuana use and later drug use.

The average age of the twins studied was 30 years old. In each set of twins studied, one had used marijuana before the age of 17. In comparison with their twins, these marijuana smokers were five times more likely to use a hallucinogen such as LSD and two times as likely to use an opiate such as heroin. Of the early marijuana users, many reported becoming dependent on marijuana (46%) or alcohol (43%). Cocaine or other stimulants (48%) were the most commonly used drugs later in life, reported the marijuana users.

The study did not address WHY this link exists, or whether there is a causal link. It is not known if smoking marijuana causes the person to try harder drugs or whether smoking marijuana somehow affects the brain in such a way that a person seeks out other drugs. It is also not known whether this link has more to do with environmental factors such as peer group, getting away with breaking the law, or access to drugs in general.

Although many marijuana users do not go on to have problems with drugs later in life, the link is strong enough that the connection represents a risk factor for future drug use.


  1. "Marijuana's link to hard drug use not genetic," by James Randerson, January 21, 2003.
  2. "Early marijuana use increases risk of drug and alcohol problems later in life," January 21, 2003.
  3. "Study of Twins Backs Marijuana-Drug Link," New York Times, January 22, 2003.
  4. "Escalation of Drug Use in Early-Onset Cannabis Users vs Co-twin Controls," Journal of the American Medical Association, January 22/29, 2003, Vol. 289, No. 4, pp. 429-433.

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