Antidepressants work by balancing chemicals called neurotransmitters that affect mood and emotions. In the 1950s, two drugs intended for other purposes - iproniazid for tuberculosis and imipramine for allergies - were found to improve patients' moods. These drugs were found to affect monoamines, leading to the hypothesis that depression results from insufficient monoamines and these drugs help restore balance by increasing monoamine availability. In the 1970s, research focused on serotonin as the key monoamine. This led to the 1988 development of Prozac, a selective serotonin reuptake inhibitor with fewer side effects. Through the 1990s, antidepressant use grew substantially. Antidepressants are often used along with psychotherapy to