 Antidepressants will often take 3–5 weeks to have a noticeable effect as the regulation of receptors in the brain adapts.There are multiple classes of antidepressants which have different mechanisms of action.Another type of antidepressant is a monoamine oxidase inhibitor (MAOI), which is thought to block the action of monoamine oxidase, an enzyme that breaks down serotonin and norepinephrine.