TUESDAY, Jan. 21, 2003 (HealthDayNews) -- Marijuana really is a "gateway" drug that leads to use of harder drugs, new research contends.
A study in the Jan. 22 Journal of the American Medical Association offers powerful new evidence that using marijuana as a teen-ager increases your risk of using harder drugs and becoming an alcoholic later in life, the researchers say.
The researchers interviewed 311 sets of same-sex twins from Australia with a median age of 30 about their marijuana use as teen-agers and their alcohol and drug use as adults. The twins were both fraternal and paternal.
Only one of each set of twins said they used marijuana between the ages of 12 and 17. But those who did substantially increased their risk of becoming dependent on alcohol or drugs later in life, the researchers say.
Twins who used marijuana before age 17 were 2.2 times as likely to become dependant on alcohol as their non pot-smoking siblings; 4.2 times as likely to try cocaine or other stimulants; 2.6 times as likely to try heroin or other opioids; and 2.2 times as likely to abuse other types of drugs.
Stated another way, about 43 percent of twins who used marijuana before age 17 went on to become alcoholics, compared to 30 percent of twins who didn't smoke pot as youths, the study says.
The study also found that:
- About 48 percent of the twins who used marijuana before age 17 became hooked on drugs later in life, compared to 33 percent of the non-pot smoking twins.
About 48 percent of the early marijuana users went on to try cocaine or other stimulants, compared to about 26 percent of twins who didn't smoke pot.
And about 13.5 percent of early marijuana users later tried heroin or other opioids, compared to 6 percent of those who didn't.
If the percentages sound high, it's because of the way the study subjects were chosen, explains Michael Lynskey, the lead author of the study and a visiting assistant professor of psychiatry at Washington University School of Medicine in St. Louis, Mo.
To be included in the study, at least one twin had to admit to having used marijuana before the age of 17.
So the results would be higher than percentages for the general population because both twins would have a higher vulnerability to drug or alcohol abuse because of shared genetic, familial, environmental and socioeconomic factors, Lynskey says.
"That's why the comparison is quite powerful," Lynskey says. "We have people exposed to the same genetic and other risk factors."
In singling out marijuana, the researchers controlled for a multitude of other factors that might make a teen more likely to try drugs. Those factors included early use of alcohol and tobacco; parental conflicts or separation; childhood sexual abuse; conduct disorder; major depression; social anxiety; early disruptive behavior; and parental drug use.
So how does smoking marijuana lead to use of other drugs?
The researchers say they don't know for sure, but it's probably involves the way marijuana works on the brain that predisposes a teen to want to try other drugs later in life.
"I think what we can say is that the association cannot be explained by familial factors, including genetic predisposition, family environment or parenting practices, which typically people will invoke as being the underlying variables," Lynskey says. "There probably is something in the drug."
However, there are other possible explanations, he adds. The researchers were unable to control for a teen-ager's friends, called the "non-shared environment," or for the environment in which the marijuana was obtained. Teens who used marijuana could have come into contact with drug dealers who made other types of drugs accessible to them, Lynskey says.
"There still has to be a little bit of caution about saying there is a causal relationship," he says.
In recent years, the "gateway" theory has come under attack, primarily from groups that support legalizing marijuana.
P>And the new study isn't making the folks at NORML (National Organization for the Reform of Marijuana Laws) rethink their position on the theory.
Study after study has found no support for the "gateway" hypothesis, says Paul Armentano, a senior policy analyst for the Washington, D.C.-based organization, which supports legalized marijuana for adults 21 and up.
"Statistically, the overwhelming majority of people who use marijuana never go on to use any other harder drugs and by their late 20s and early 30s have stopped using marijuana all together," Armentano says. "So looked at it that way, marijuana is actually a terminus."
And there is even less proof that there is something pharmacological in marijuana that makes a person crave other drugs, he adds.
"There is nothing unique about the pharmacology of marijuana to say that it somehow primes their brain to have a greater preference for drugs," Armentano says. "Heroin and cocaine work on the body and the brain in dramatically different ways."
However, he does agree that a black-market environment in which marijuana is purchased could contribute to widened exposure to other drugs and people's use of them.
While the link between teenage pot smoking and later drug use may scare parents, Lynskey says, it's important to remember that the majority of marijuana smokers in the study didn't go on to become drug addicts or alcoholics.
"If you are a parent of a 16-year-old who's using cannabis, it's not inevitable they will go on to use other drugs," he notes.
In 1999, there were 220,000 marijuana-related admissions to publicly funded substance-abuse treatment programs in the United States, according to the study. This represented 14 percent of all treatment admissions.
About one-third of all marijuana-related admissions were for youths between 12 and 17 years old.
More information
The National Institute on Drug Abuse has information for teens on marijuana and its affects on the body. For help dealing with a marijuana addiction, visit Marijuana Anonymous.
