Selegiline is a drug used for the treatment of early-stage Parkinson's disease and senile dementia. In normal clinical doses it is a selective MAO-B inhibitor. It is sometimes used off-label to treat narcolepsy and as a nootropic, as well as for its published life-extending effects among several species of mammals. It is also reported to positively affect libido, particularly in older males. Selegiline is also used to treat major depression using a transdermal patch and it is also used (at extremely high dosages relative to humans) in veterinary medicine to treat the symptoms of Cushing's disease and so-called "cognitive dysfunction" in dogs. Selegiline is a selective inhibitor of MAO-B; MAO-B metabolizes dopamine. Selegiline exhibits little therapeutic benefit when used independently, but enhances and prolongs the anti-Parkinson effects of levodopa.