However, they're the most effective treatment in relieving symptoms quickly, particularly in cases of severe depression.
The Royal College of Psychiatrists estimates that 50-65% of people treated with an antidepressant for depression will see an improvement, compared to 25-30% of those taking inactive "dummy" pills (placebo).
They may recommend increasing your dose or trying an alternative medication.
A course of treatment usually lasts for six months, although a two-year course may be recommended for people with a previous history of depression.
This is why they're usually used in combination with therapy to treat more severe depression or other mental health conditions caused by emotional distress.
If you take an antidepressant for four weeks without feeling any benefit, speak to your GP or mental health specialist.
Increasing levels of neurotransmitters can also disrupt pain signals sent by nerves, which may explain why some antidepressants can help relieve long-term pain.
While antidepressants can treat the symptoms of depression, they don't always address its causes.
Antidepressants are also sometimes used to treat people with long-term (chronic) pain. It's thought that antidepressants work by increasing levels of a group of chemicals in the brain called neurotransmitters.
Certain neurotransmitters, such as serotonin and noradrenaline, can improve mood and emotion, although this process isn't fully understood.
Some people with recurrent depression may be advised to take them indefinitely. Different antidepressants can have a range of different side effects.