Jan. 21, 2003 -- It's commonly believed that early drug use -- widely considered the biggest indicator of later abuse -- is somehow "hot-wired" to genetic makeup and largely influenced by a troubled home life. But a new study suggests that DNA and mom and dad can't get all the blame.
In the 1960s, young twins were enlisted by Australian researchers as part of a massive effort to learn how genetics influence a range of behaviors, including drug use. Of the 6,000 twins originally recruited, the researchers identified 311 sets in which one sibling used marijuana before age 17 while the other did not, including 136 sets of identical twins and 175 sets of fraternal twins. They were surveyed again between 1996-2000.
The researchers report that those twins who first smoked pot before age 17 were two to five times more likely to continue using other drugs or alcohol or develop drug-related problems than their siblings who were drug-free in high school.
Each twin pair studied lived in the same home during adolescence. And yet, the findings -- reported in the Jan. 22/29 issue of TheJournal of the American Medical Association -- depart from previous studies, including at least one funded by the National Institute on Drug Abuse, that suggest the progression to later abuse or dependence was "due largely to genetic factors."
"We were certainly surprised by our findings," says study researcher Michael Lynskey, PhD, a senior research fellow at Queensland Institute of Medical Research in Australia who is a visiting professor at the Washington University School of Medicine in St. Louis. "I expected that after we controlled for genetics and a common family environment, the association between early cannabis use and later risks of other drug use and drug-related problems would be reduced. It wasn't."
And he's not the only one surprised by the finding.
"This is surprising, given the results found in previous studies," says Carol A. Prescott, PhD, whose own research found that early alcohol abuse in one teenaged twin predicted a quick and similar result in the other. "When you look at the study participants who used marijuana early, it seems to be that individual factors -- not family, genetics or environment -- were what predicted who would get to the next stage of drug use. Still, you can't throw genetics out of the window."
Though early-age use was a clear predictor of later drug-related problems in Lynskey's study, he notes that those who left high school drug-free didn't necessarily stay that way. Though 52% of early pot smokers continued smoking pot to some degree into adulthood, they were joined by 43% of the "drug-free" twins. Meanwhile, 40%-50% of early users and almost 25%-35% of later starters used LSD or other hallucinogens or cocaine.
"It is important to know that age of first use isn't the only risk factor," Lynskey tells WebMD. "There are multiple factors associated with increased risks -- including genetics, social disadvantage, family factors such as divorce, parenting style. But there is also peer influences and a choice that is made by some (youths) to start using drugs."
"Although people who use cannabis at early age are at an increased risk at later drug use, parents should know that it is not the case that they will inevitably use drugs or develop problems," Lynskey tells WebMD. "In fact, the majority of them don't. So if you're a parent and find a joint on your 15-year-old, this is probably something you want to hear."