The study appears in October's edition of Alcoholism: Clinical and Experimental Research.
Nearly 600 youths aged 12-17 years answered questions about their use of alcohol, tobacco, or other drugs.
One of their parents also participated; more than eight out of 10 parents were mothers. The parents were asked if they thought their child had tried or abused any of the substances.
The results show that many parents were in the dark about their teen's substance use, especially with the youngest adolescents (aged 12-13 years).
That finding is "very troubling because research has shown that starting to use alcohol and drugs at a young age is a risk factor for developing substance abuse or dependence in the future," says researcher Laura Bierut, MD, in a journal news release.
Bierut works in St. Louis at Washington University's psychiatry department.
Teen Said, Parent Said
Across the board, parents underestimated adolescent substance use.
For instance, more than half of the teens -- 54% -- said they had ever had an alcoholic drink, and nearly a quarter said they had ever gotten drunk.
But only 30% of parents said their teen had ever had a drink, and only 8% said their child had ever gotten drunk.
Also, more than four in 10 teens said they had ever used tobacco, and more than one in five said they had ever used marijuana.
But less than three in 10 parents said their teen had ever used tobacco, and about 13% said their teen had used marijuana.
In short, "55% of adolescents who had smoked cigarettes, 50% who had used alcohol, and 47% who had used marijuana had a parent who knew that they used" those substances, the researchers write.
Parents were even less aware of teen use of other drugs, such as cocaine.
About 8% of teens said they had used such drugs, while only 3% of their parents said they knew about that drug use.
Parents of older teens were more likely to acknowledge adolescent alcohol, tobacco, and drug use.
It's possible that adolescent substance use was exaggerated by some teens and downplayed by some parents.
Still, the findings may serve as a wake-up call to some parents.
"Although as parents we might not like to think that our children are not reflected in these percentages, it is important to realize that our kids do have access to substances and might very well be using them," Bierut says.
SOURCES: Fisher, S. Alcoholism: Clinical and Experimental Research, October 2006; vol 30: pp 1-12. News release, Alcoholism: Clinical and Experimental Research.
By Miranda Hitti
Reviewed by Louise Chang