Monoamine oxidase inhibitors (MAOIs) are used to treat depression and Parkinson's disease. MAOIs work by blocking the enzyme (protein) monoamine oxidase, which breaks down neurotransmitters (chemical messagners) like serotonin, dopamine, and norepinephrine, thereby increasing their levels in the brain. These medications come in different forms, including oral pills and transdermal patches. Examples include selegiline (Emsam), rasagiline (Azilect), and phenelzine (Nardil). These medications have several drug interactions, so talk to your prescriber before starting a new medication if you take an MAOI.