Anti-depressants have been prescribed since the 1950s, and their prevalence has significantly increased since then. There are many classes of anti-depressant pharmaceuticals, such as selective serotonin reuptake inhibitors (SSRIs), monoamine oxidase inhibitors (MAOIs), and tricyclic anti-depressants. Many of these drugs, especially the SSRIs, function by blocking the metabolism or reuptake of neurotransmitters to treat depression and anxiety. Chronic exposure or overdose of these pharmaceuticals can lead to serotonin and CNS hyperexcitation, weight changes, and, in severe cases, suicide.