According to scientists, antidepressants such as Prozac work because they increase serotonin, a neurotransmitter which makes us happy.
Unfortunately, conflicting evidence may discount this theory.
Firstly, antidepressants only work on just over half of patients. Furthermore, since serotonin cures depression, then scientists should then be able to induce depression by lowering the amount of serotonin in the brain. In repeated experiments, this hasn’t worked; it sometimes lowers mood, but it cannot come close to inducing depression.