Antidepressants are a class of medications used to treat major depressive disorder, anxiety disorders, chronic pain, and addiction.

Common side effects of antidepressants include dry mouth, weight gain, dizziness, headaches, akathisia, sexual dysfunction, and emotional blunting. There is an increased risk of suicidal thinking and behavior when taken by children, adolescents, and young adults. Discontinuation syndrome, which resembles recurrent depression in the case of the SSRI class, may occur after stopping the intake of any antidepressant, having effects which may be permanent and irreversible. Tapering off medications gradually is shown to reduce the risk of withdrawal complications.

The effectiveness of antidepressants for treating depression in adults has strong support, though studies also highlight potential risks and limitations. In children and adolescents, evidence of efficacy is more limited, despite a marked increase in antidepressant prescriptions for these age groups since the 2000s. A 2018 meta-analysis reported that the 21 most commonly prescribed antidepressants were found in all studies to be more effective than placebos for the short-term treatment of major depressive disorder in adults. However, other research suggests that some benefits may be attributable to the placebo effect. Response to antidepressants is highly variable, and medications that are effective for certain patients may have no effect or a negative effect for others. Research into the factors that influence individual responses to antidepressants is ongoing. ‌

Antidepressants are prescribed to treat major depressive disorder (MDD), anxiety disorders, chronic pain, and some addictions. Antidepressants are often used in combination with one another.

Despite its longstanding prominence in pharmaceutical advertising, the idea that low serotonin levels cause depression is not supported by scientific evidence. Proponents of the monoamine hypothesis of depression recommend choosing an antidepressant which impacts the most prominent symptoms. Under this practice, for example, a person with MDD who is also anxious or irritable would be treated with selective serotonin reuptake inhibitors (SSRIs) or norepinephrine reuptake inhibitors, while a person suffering from loss of energy and enjoyment of life would take a norepinephrine–dopamine reuptake inhibitor.

The UK National Institute for Health and Care Excellence (NICE)'s 2022 guidelines indicate that antidepressants should not be routinely used for the initial treatment of mild depression, "unless that is the person's preference". The guidelines recommended that antidepressant treatment be considered:

The guidelines further note that in most cases, antidepressants should be used in combination with psychosocial interventions and should be continued for at least six months to reduce the risk of relapse and that SSRIs are typically better tolerated than other antidepressants.

American Psychiatric Association (APA) treatment guidelines recommend that initial treatment be individually tailored based on factors including the severity of symptoms, co-existing disorders, prior treatment experience, and the person's preference. Options may include antidepressants, psychotherapy, electroconvulsive therapy (ECT), transcranial magnetic stimulation (TMS), or light therapy. The APA recommends antidepressant medication as an initial treatment choice in people with mild, moderate, or severe major depression, and that should be given to all people with severe depression unless ECT is planned.