Phelps lost a lucrative sponsorship after being photographed smoking pot. He was diagnosed with ADHD as a child.(GETTY IMAGES)

After a British tabloid photo of Michael Phelps apparently smoking marijuana at a college party surfaced early this month, the Olympic swimmer apologized to his fans and to the public, citing his youth, his “regrettable” behavior, and his “bad judgment.” What he didn't mention—and what may or may not have influenced his behavior—was attention deficit hyperactivity disorder (ADHD), a condition with which he was diagnosed at age nine.

About 60% of children with ADHD have symptoms that persist into adulthood. Phelps no longer takes medication for ADHD, and his mother has said he is now able to focus his attention using physical training (including swimming) and behavioral modification he learned as a child.

While the 14-time Olympic gold medalist is generally seen as an all-American role model, this most recent photo was not the first blemish on his record: In November 2004, Phelps—just 19 at the time—ran a stop sign and was arrested for driving under the influence.

More about adult ADHD

In recent days, several blogs have raised Phelps ADHD in connection with the party photo, suggesting that he may have been "self-medicating"; that lots of people with ADHD smoke pot; or, at the very least, that they tend to act before thinking. Other commentators have suggested that Phelps is simply a normal 23-year-old cutting loose after years of rigorous training and self-discipline.

No one can say if ADHD played a role in Phelps behavior. However, the episode does serve as a reminder that there are unanswered questions about ADHD's impact on impulsive decision making and substance use—and the importance of seeking diagnosis and treatment as soon as possible, before problems develop.

[ pagebreak ]Research suggests that children with ADHD are more likely than their non-ADHD peers to drink alcohol and use drugs later in life, specifically as teenagers. A 2003 study published in the Journal of Abnormal Psychology found that more than a third of the ADHD participants (out of 140 teenagers who had been diagnosed as children) reported smoking marijuana, compared to just over a quarter of the non-ADHD control group. Nearly twice as many in the ADHD group reported being drunk more than once in the previous six months.

For children whose ADHD continues into adulthood, the overall risk of developing a substance use disorder is as high as 50%, by some estimates. (Substance use disorders include a range of problematic drinking and drug-use patterns, not dependence or addiction alone.)

Inattention and impulsivity both play a role

Although the exact connection remains unknown, experts believe ADHD and substance use are linked by a complex blend of environmental and genetic factors.

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"Some of the risk factors for ADHD are also risk factors for substance abuse," says Stephen Faraone, PhD, a professor of psychiatry and behavioral sciences at SUNY Upstate Medical University. "The apparent genetic link between ADHD and substance abuse is pretty well validated by family studies and twin studies."

Several questions about the link between ADHD and substance use remain unanswered, however. It is unclear, for instance, whether the symptoms of ADHD—most notably impulsivity—lend themselves to substance use (and abuse) or whether negative effects of those symptoms (such as difficulty with school, work, or relationships) steer people toward drinking and drug use.

In the 2003 study, the use of drugs and alcohol was correlated most strongly to the adolescents' levels of inattention, which, along with impulsivity-hyperactivity, is considered one of the main characteristics of ADHD.

This finding was somewhat surprising, says study coauthor Brooke Molina, MD, an associate professor of psychiatry at the University of Pittsburgh School of Medicine. Researchers who study substance use and ADHD have tended to "zero in on impulsivity," she explains, because studies have shown impulsivity can predict alcohol, drug, and tobacco use over the long term.

Although impulsivity clearly plays an important role in ADHD and substance use, Dr. Molina suspects that inattention—which can cause academic and social problems that can in turn contribute to substance use—may prove to a more important factor than has been thought.

Next Page: Job and relationship stress may affect ADHD adults

[ pagebreak ]The role of age in substance use among people with ADHD is also an open question. Most studies have looked at teenagers, rather than adults in their 20s and beyond,so the ways in which ADHD affects substance use in different stages of life remain unclarified.

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In a 2007 study, Dr. Molina and her coauthors reported that childhood ADHD predicted heavy drinking in 15- to 17-year-olds, but not in 18- to 25-year-olds. It's not that the young adults with ADHD suddenly stopped drinking, Dr. Molina explains; it's that they entered the age bracket, which includes college students, in which frequent and heavy drinking is most common.

Dr. Molina and her colleagues plan to follow the study participants into their late 20s, when heavy drinking in the general population tends to subside as people marry and pursue careers. But adults in that age range with ADHD, she notes, tend to experience difficulties with job performance, social relationships, and other common life experiences.

"We have a hypothesis that a substantial number of those ADHD heavy drinkers will not mature out of it," Dr. Molina says. "There are quite a few impairments that will follow a number of these kids that should in theory contribute to continued drinking problems."

[ pagebreak ]Phelps was diagnosed with ADHD in elementary school, Phelps's mother told the New York Times in 2008, when his teachers remarked on his lack of focus and inability to sit still. Between the ages of 9 and 11, he took Ritalin, a stimulant medication prescribed for ADHD. Ultimately, he asked to be taken off the medication because he felt stigmatized by going to the nurse's office every day.

Treatment Options for ADHD at Any Age

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You may consider medication, talk therapy, or lifestyle changes Read moreMore about Adult ADHD

Some research suggests that children with ADHD who are treated with stimulants may be less likely to use drugs and alcohol. (Some observers, on the other hand, have suggested that stimulants, which can be abused, may actually predispose children with ADHD to substance use.)

A 2008 study that followed a group of adolescent women with ADHD for five years found that the participants who had been treated with stimulants were nearly 75% less likely to develop a substance-use disorder than those who were not, while other research has shown that the use of ADHD medication in young men reduces the risk of later substance-use disorders by 85%.

Although the reason for this apparent "protective effect" remains unknown, Faraone (who was involved in both of the studies) says the symptoms of ADHD seem to play a role. "It's very possible that symptoms such as impulsivity put adolescents at risk for substance abuse," Faraone says. "If you're an impulsive adolescent, and you're at a party where someone is passing around a marijuana cigarette, you're more likely to say 'I'll try that' if your impulsive symptoms are not controlled well by stimulants."

Is the ADHD brain drawn to drugs and alcohol?

Another explanation may be genetic. Both stimulant treatment and substance use involve dopamine and catecholamine, chemicals in the brain involved in pleasure, reward, and stress.

Studies in hyperactive rats have shown that stimulant therapy decreases hyperactivity as well as the appetite for alcohol and other substances, according to Faraone, which suggest that stimulants may act on ADHD symptoms and substance use in similar ways.

"A medication like a stimulant affects the way the brain works," Faraone says. "And when you affect the way the brain works in a child, what you're doing in the child's brain to help ADHD may somehow also be preventing substance abuse at a neurological level, as opposed to a behavioral level."