Parents may be unaware as much as half of the time that their teenagers are using drugs or alcohol, say researchers who compared reports from both generations.

While previous research has suggested parents can be helpful in reporting symptoms of attention deficit hyperactivity disorder (ADHD), the same doesn't seem to hold true for alcohol and drug abuse, the team said.

Laura Jean Bierut, a professor of psychiatry, and her colleagues at Washington University's school of medicine in St. Louis, Mo., compared interviews from nearly 600 pairs of adolescents and their parents who were participating in a study on the genetics of alcoholism.

The researchers, reporting in the October issue of the journal Alcoholism: Clinical & Experimental Research, said the most commonly used substances according to the teens were:

• Alcohol, 54 per cent.
• Tobacco, 44 per cent.
• Marijuana, 23 per cent.

Among children who reported using alcohol, the parents reported it only 50 per cent of the time, the study showed. When children said they used tobacco, their parents reported it 55 per cent of the time, and for marijuana users, the parents reported it 47 per cent of the time.

Parents seemed even more unaware of a child's use of harder drugs. For children using drugs such as cocaine or methamphetamine (speed), for example, the parents reported use in only 28 per cent cases.

"Our conclusion is that parents do not provide valuable information about their children's use of alcohol and drugs because they simply don't know about it," said study co-author Sherri Fisher.

Even among children who reported experiencing symptoms of dependence with alcohol or marijuana, there were parents who were unaware their offspring had used the substances, the researchers found.

Symptoms less clear

Bierut said "externalizing" disorders such as ADHD may show clearer behaviours, such as a child who cannot sit still or argues with parents, while the symptoms of "internalizing" disorders such as depression are not recognized as easily.

"Things like feelings of worthlessness or loss of interest in favourite activities can be very troubling to a child, but they don't necessarily impact others and might go unnoticed unless the child chooses to talk about them," Beirut said.

Parental recognition of adolescent substance abuse was highest among parents of teens who were 16 or 17, and lowest among parents of youths aged 12 or 13, the researchers report in the October issue of Alcoholism: Clinical & Experimental Research.

"This 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," Beirut said.