My doctor put me on 20 mg Fluoxetine (Prozac) for severe PMS. It was great, but I was having migraines which I never had before. They wanted to try something else just in case, so they put me on 37.5 Venlafaxine (Effexor). The doctor said don't stop, just switch the next day to the next drug. I want to go back to Fluoxetine. I did more research on Effexor and want to go back. I should have done it before, I know. Has anyone just switched back before? Is it dangerous to do?