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Antidepressants are medications used to treat major depression and a number of other disorders (e.g., personality disorders, anxiety disorders). They are not habit-forming and are safe if used as directed by a physician. They are generally taken in pill form, often once or several times daily. However, extended-release and controlled-release versions are available for many antidepressants. These can be taken less frequently.

Nerves send messages to each other by means of neurotransmitters. Large numbers of neurotransmitters are released for each message. Many neurotransmitters adhere to receptors on other nerves. These receptors read the messages. Some neurotransmitters are brought back into the nerve cells without adhering to receptors and being read. This process, which is called reuptake, allows the nerves to reuse neurotransmitters instead of continually making more.

Antidepressants appear to work by increasing the levels of the neurotransmitters serotonin, norepinephrine and dopamine (collectively called monoamines) in the synapses (space between the nerves) of the brain. Monoamines are involved in thought, emotions, memory and the regulation of certain body functions. When the levels of these chemicals are too low, patients may become depressed.

Levels of monoamines can be increased by blocking the receptors that read the messages (receptor blockers) or blocking the reuptake of the chemicals (reuptake inhibitors). When either the reuptake or the receptors are blocked, the levels of the monoamines in the brain are increased.

The effect of antidepressants on brain chemistry may not be immediate. It may take several weeks for a patient to begin to respond to the medication. Initially, only side effects (e.g., insomnia, dry mouth, fatigue) may occur. In most cases, the side effects improve or go away after about a week or two, and newer antidepressants have fewer side effects than older antidepressants.

It is important for a patient to take an antidepressant for at least six months. This allows the medication to have time to work and reduces the risk of relapse. Antidepressants may be taken for months or years depending on the condition that is being treated.

Antidepressants need to be taken exactly as prescribed in order to provide their desired effect. Skipping a dose may cause these medications to not work properly. It is also important not to stop taking antidepressants without consulting a physician. Many people confronted with initial side effects and no immediate positive effect stop taking antidepressants before they have had a chance to work. When antidepressants are discontinued suddenly, they may cause withdrawal symptoms (e.g., nausea, headache, dizziness). Such symptoms can usually be prevented by stopping antidepressant use gradually.