Amphetamine is also prescribed in enantiopure and prodrug form as dextroamphetamine and lisdexamfetamine respectively. Lisdexamfetamine is structurally different from amphetamine, and is inactive until it metabolizes into dextroamphetamine. Dextroamphetamine is useful for those with ADHD and Narcolepsy. It improves self-control for people who have a hard time naturally controlling themselves. Dextroamphetamine aids a person learning and memory of words, and perhaps makes the brain stronger. When a person given dextroamphetamine is tested, their brain is extremely active in the brain parts required for the test and radically less active in other parts. Short practice sessions with dextroamphetamine have a greater effect on learning than sessions without dextroamphetamine. Dextroamphetamine raises decision-making scores, improves choices, and changes beliefs about rewards; at the same time, dextroamphetamine barely—if at all—affects guesses of time. Those who feel lower amounts of joy from dextroamphetamine have greater impulsivity improvements compared to those who feel extreme happiness. The drug should be avoided for those who have hypersensitivity to amphetamines, a history of drug abuse, cardiovascular diseases, hypertensive disease, hyperthyroidism, or in those with glaucoma. In 1935, the medical community became aware of the stimulant properties of amphetamine, specifically dextroamphetamine, and in 1937 Smith, Kline, and French introduced Dexedrine tablets, under the tradename Dexedrine. In the United States, Dexedrine tablets were approved to treat narcolepsy, attention disorders, depression, and obesity. Dexedrine, along with other sympathomimetic, was eventually classified as schedule II, the most restrictive category possible for a drug with recognized medical uses. The exact mechanism of action is not known. Dextroamphetamine stimulates the release of norepinephrine from central adrenergic receptors. At higher dosages, it causes release of dopamine from the mesocorticolimbic system and the nigrostriatal dopamine systems by reversal of the monoamine transporters. Dextroamphetamine may also act as a direct agonist on central 5-HT receptors and may inhibit monoamine oxidase (MAO). Modulation of serotonergic pathways may contribute to the calming effect.