The amount of time an antidepressant stays in the body depends on the medication. Some medications are 99 percent out of the body in around one day while others, such as Prozac, can take up to 25 days to leave the system.
Drugs containing fluoxetine, which includes Prozac, stay in the body the longest. Their half-life or the amount of time it takes for half the medication to leave the body is four to six days and it will usually stay in the body for close to a month after a person stops taking it. Drugs containing venlafaxine, which include Effexor have a half-life of five hours and will be completely out of the body in one day.