Adderall is an amphetamine known for treating attention-deficit hyperactivity disorder (ADHD). But the high number of players taking Adderall has led Major League Baseball to designate it as a performance-enhancing drug and sharply limit its use.

Adderall first became available in 1996 as an instant-release tablet. Five years later, Adderall XR - an extended-release form - came on the market. Both are available in generic forms.

Adults with ADHD often struggle with planning, impulse control, and being easily distracted. Symptoms typically manifest before age 7, but can remain long into adulthood.

Adderall has been shown to significantly reduce ADHD effects, albeit with side effects that can include dizziness, headache and weight loss.

Adults without ADHD who take Adderall can enjoy these same attention-span increasing benefits. In 2011, a total of 105 Major League baseball players were granted exemptions under the league's drug policy to fill Adderall prescriptions. That's about one in every 10 players, a much higher rate than the general population.

"The anticipated benefit is that performance and stamina can be enhanced," said Daniel Hussar, professor of pharmacy at the Philadelphia College of Pharmacy at the University of the Sciences. "But sometimes individuals get into such a habit of using these medications that it can be quite a letdown when they stop using them. They feel compelled to continue using the product."

Earlier this year, MLB tightened criteria for such exemptions, responding to the high rate of prescriptions among its players. Since last June, players receiving exemptions for Adderall or other stimulants need the approval of a three-expert panel. In the past, only one person had ruled on such requests.

Adderall is not a known masking agent for other performance-enhancing substances, Hussar said. But he feels strongly that MLB should regulate Adderall use by its players.

"These are potent medications that can have serious consequences if misused," he said. "If individuals let their guard down with these substances, circumstances can increase the risk of damage to the cardiovascular system."