Antidepressant drugs are effective for about three in four people with depression. For reasons that are not understood, individual patients who do not respond to one drug often respond to another. Differences in mechanisms of action may thus be important in determining treatment success or failure. In addition to efficacy, drug side effect profile also determines treatment outcome. In general, the fewer or less severe the side effects of a drug, the greater the degree of compliance with treatment. A major consequence of the introduction of selective serotonin-specific antidepressants is greater patient acceptance due to fewer side effects. Still, some patients are unable to tolerate the nervousness, insomnia, or sexual dysfunction associated with these drugs. Drugs that are even more specific in that they act on specific serotonin receptor subtypes, rather than only by blocking serotonin uptake, may provide efficacy and fewer side effects for patients who do not respond to or tolerate less specific agents.