When my oldest son was 13 he began to do poorly in school. He often forgot to do his homework, or he did it but forgot to hand it in. He had difficulty paying attention during tasks, was extremely disorganized, and his teachers said he was easily distracted. They suggested that his behavior fit with many of the symptoms of attention deficit hyperactivity disorder (ADHD), but since this was mostly new behavior, there was some doubt in my mind.

Also new: my son had just transferred to a different middle school, and while he was having trouble adjusting to the change in routines, he was also making many new friends and exploiting his sudden popularity.

When a child’s school performance deteriorates during their teens, a whole host of questions arise. Is he just going through a phase? Is it ADHD? Could it be alcohol, or maybe, drugs? The Consumer Reports National Research Center survey on ADHD, which asked 934 parents to share their observations, found these to be some of the top characteristics of ADHD behavior: easy distractibility (95 percent), difficulty focusing or maintaining attention during tasks (95 percent), quickly losing interest (91 percent), and not following through with schoolwork or chores (91 percent).

Many of the signs of teen drug abuse, described in Time to Act!, a Web site produced by the Partnership for a Drug-Free America, are strikingly similar: loss of interest in school work , drop in grades, apathy about extracurricular activities, hobbies or sports, failure to fulfill responsibilities at school, complaints from teachers, decreased motivation, inability to focus, hyperactivity, and making endless excuses.

To make matters even more confusing, studies indicate that there is a high rate of substance abuse in teens with ADHD, which can complicate the management of both conditions. Affected adolescents start doing drugs earlier, are likely to have poorer outcomes, and have higher rates of other psychiatric disorders. What this means is that if you’re the parent of a teen with ADHD, it’s important to be able to detect the signs of substance abuse.

But how can a parent distinguish between the two?

For one thing, in ADHD the pattern of behavior usually begins before age 7, although the disorder may be diagnosed at a much later time. In our survey, the average age at diagnosis was 7 1/2. Although the diagnostic workup may not take place until an older age, as with my son, parents can usually look back and figure out if the traits were present at an earlier age.

Children with ADHD are usually keenly aware, even frustrated, by their struggles, while those abusing drugs often appear apathetic about their performance and suddenly indifferent to their favorite extracurricular activities or sports. Drug abuse experts point out additional behaviors that can serve as tip-offs: Sudden use of eye drops to decrease eye reddening, or breath mints/gum to cover up bad breath, appetite changes, poor hygiene, anger, hostility, sullenness, withdrawal, secretiveness, and emotional volatility. The physical signs include the inability to speak intelligibly, slurred speech, frequent nosebleeds or runny nose, wetting of the lips or frequent thirst, sudden or dramatic weight loss or gain, skin abrasions/bruises, sweatiness, headaches or vomiting.

I watched my son pretty closely over those years, but aside from the school problems didn’t find much else to arouse suspicion. I never noticed symptoms of drug use, and like 14 percent of parents in our survey, I was not convinced my child had ADHD. But at the school’s suggestion, I took him to see a specialist who, while also uncertain about the diagnosis, suggested a trial of medication. As a neurologist, I’d seen many patients improve dramatically on drug therapy, but it didn’t seem to make a difference in my son’s behavior. He did, however, benefit from extra help provided during and after school and from learning organizational strategies.

Eventually, things began to click for him and he began to show an interest in academics. Did he grow out of it or did he just grow up? I’ll probably never know for sure, but by the end of his sophomore year in high school, he was working independently enough to be accepted into an early college program. Today at 18, he’s entering his junior year.

If you’re concerned about your child’s behavior, I encourage you to make use of the resources available to parents, both inside the schools and elsewhere. Our report on ADHD provides guidance for parents who are new to the world of ADHD diagnosis and treatment, including parents’ real life assessments of which practitioners and therapies are most helpful.

Orly Avitzur is medical adviser at Consumer Reports and blogs at the Consumer Reports Health Blog.

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