Prozac is a potent SSRI (serotonin reuptaker ) so some people don't do well at the higher 40 mg dosage. Some people only need 20 mg. Also it takes a long time for Prozac to get into your system and some docs don't wait long enough before they up the dosage. It is up to your symptoms that you are having on the higher dosage. It is best to slowly be increased up to 40 mg. So let your doc know what your symptoms are. Best of luck.