Antidepressants affect several chemicals in the brain (neurotransmitters) that influence mood and anxiety, including:
serotonin
noradrenaline
dopamine
melatonin
Because of how they work, you may need to take antidepressants for a while before you see your symptoms improve.
If you have depression, you may begin to feel better 2 to 4 weeks after starting an antidepressant medication, but it can take up to 6 to 8 weeks to feel the full effect.
If you have an anxiety disorder, it may take 4 to 6 weeks before you start feeling better. It may take up to 12 weeks to feel the full effect.
In some people, antidepressants are not effective, no matter how long they take them for.