The Turkish basketball federation said Friday that the star player Diana Taurasi had tested positive for a stimulant that was identified as modafinil, a drug that enhances wakefulness and vigilance and is designed to treat narcolepsy, sleep apnea and sleep disorders related to shift work. Modafinil is also used off-label to combat jet lag and attention deficit disorder and recreationally to enhance alertness. The stimulant does not cause the anxiety and jitteriness associated with amphetamines.

Athletes who use modafinil as a banned performance-enhancing substance, however, face suspension from competition for up to two years, according to the rules of the World Anti-Doping Agency. That could keep Taurasi from playing in the 2012 Olympics.

Taurasi’s case could still be declared a false positive and dismissed. But even if it were determined that Taurasi had taken the stimulant inadvertently, and a two-year ban were reduced, she could still face the loss of her Olympic eligibility.

The International Olympic Committee instituted a controversial rule in 2008 that prohibits athletes from competing in the next Winter or Summer Games if they have served a doping suspension of six months or longer.