I have been on different doses of Prozac since August trying to get it right. I think it stays even in the system for about 7 weeks or so, but some people lose the effects faster than others. I was on 30mg and was supposed to drop to 15 but dropped to 10 by mistake, so it makes sense that I was having more symptoms this past week along with all the stress in my house. Fortunately, I don't have any bad side effects of taking Prozac. it is one of the few meds I have found that agree with me.