Half life is how long the drug stays in your systen. That is why you feel crummy if you ever missed a pill because effexors half life is so short. Where prozac has a half life of five days most are only hours. That’s why they also came up with extended release. Effexor XR. They often use prozac in low dose form to manage the weening of Effexor that is what saved me in the beginning going off Effexor.