We all react differently to anti depressants. I had to come off Prozac after 4 weeks as it was making me so ill. Opposite to you though, I was hyper, running around crying. Could not keep still. I was on 10mg. Yuk never again.
You may find after a time the foggy head will go. It may be that Prozac does not suit you either.
over a month, but only close to 3 weeks at 20 mg
How long have you been taking it?