Eigenpoll : Cognitive improvement : Amphetamines 

from http://en.wikipedia.org/wiki/Amphetamine

Amphetamine is used to treat attention deficit hyperactivity disorder (ADHD) and narcolepsy, and is sometimes prescribed off-label for its past medical indications, such as depression, obesity, and nasal congestion. Long-term amphetamine exposure in some species is known to produce abnormal dopamine system development or nerve damage, but, in individuals with ADHD, stimulants appear to improve brain development and nerve growth. Magnetic resonance imaging studies suggest that long-term treatment with amphetamine decreases abnormalities in brain structure and function found in subjects with ADHD, and improves function of the right caudate nucleus and other parts of the brain involved in dopamine transmission.