Selegiline is a selective inhibitor of MAO-B, irreversibly inhibiting it by binding to it covalently. It is generally believed to exert its effects by blocking the breakdown of dopamine, thus increasing its activity; however, recent evidence suggests that MAO-A is solely or almost entirely responsible for the metabolism of dopamine. Its possible neuroprotective properties may be due to protecting nearby neurons from the free oxygen radicals that are released by MAO-B activity. At higher doses, selegiline loses its selectivity for MAO-B and inhibits MAO-A as well.
Selegiline potentiates the release of catecholamines independent of its MAO-B inhibiting action. As such, it has been called the "first synthetic catecholaminergic activity enhancer substance".
Selegiline also inhibits CYP2A6 and can increase the effects of nicotine as a result. Selegiline also appears to activate σ receptors, having a relatively high affinity for these receptors of approximately 400 nM.