Overview
Selegiline is a monoamine oxidase type B (MAO-B) inhibitor used primarily as an adjunct therapy in the treatment of Parkinson’s disease. It helps to conserve the amount of dopamine available in the brain by preventing its breakdown, thereby prolonging the effects of levodopa.

Mechanism of Action
It selectively and irreversibly inhibits MAO-B, the enzyme primarily responsible for the degradation of dopamine in the brain. By blocking this enzyme, selegiline increases dopamine levels in the striatum and enhances and prolongs the anti-Parkinsonian effects of levodopa. At recommended doses, it does not significantly inhibit MAO-A, reducing the risk of a hypertensive crisis associated with dietary tyramine.

Dosage and Administration
The typical oral dose is 5 mg taken twice daily, usually at breakfast and lunch. Taking it late in the day should be avoided as it can cause insomnia. An orally disintegrating tablet and a transdermal patch (primarily for depression) are also available.

Side Effects
Common side effects include nausea, dizziness, insomnia, and dry mouth. When used with levodopa, it may exacerbate levodopa-induced side effects such as dyskinesias and hallucinations, requiring a reduction in the levodopa dose.

Contraindications
It is contraindicated in patients with a known hypersensitivity to selegiline. It should not be used concurrently with meperidine, tramadol, methadone, or other MAO inhibitors due to the risk of severe and potentially fatal reactions (serotonin syndrome).

Pregnancy and Lactation
It should be used during pregnancy only if clearly needed. It is not known whether selegiline is excreted in human milk, so caution is advised when administering to a nursing woman.