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Lexapro and Prozac are both antidepressants that work as Selective Serotonin Reuptake Inhibitors. These medications work on serotonin, which plays a role in both depression and anxiety. Lexapro is usually a well tolerated medication with few side effects. However some people just have bad side-effects (like nausea, increased anxiety, sexual side-effects, etc.). If a person with Bipolar
disorder takes SSRIs they can have a manic episode followed by depression. If a person had no mental disorder and took SSRIs they would probably feel no effect, unless they were taking a high enough dose to overwhelm the body which could cause serotonin syndrome.
Was there a reason why the doctor wanted to try something different, when you'd had success with Prozac in the past? Usually if a medication did
it's job and was well tolerated, doctors recommend using the same one again. If, however, there were bad side effects, doctors would want to try something new to try to avoid that.
If there was no reason to leave Prozac, you might want to tell your doctor that because of these side effects you'd like to try it again. If there was a reason to try something else, then you might need a lower dose. I have seen people who were very sensitive to Lexapro and needed a smaller than expected dose (as low as 5mg/day instead of the 20/mg that is usually seen as normal for adults). If the negative side effects are still too strong on a lower dose of Lexapro, then it's time to try a different antidepressant.