Efficacy of low dose of prozac

I have been taking prozac for two years now, I started by building up to 20mg per day and on that dose although I recovered enough to lead a normal life after a while I felt a bit spaced out.

I gradually reduced my dose to 2.5mg per day. Everything I have read suggests the therapeutic dose range is 10mg/d to 80mg/d, so I stopped taking it. I started to get the anxiety symptoms back after a couple of months. So now I am back on 2.5mg/d (or 5mg on alternate days as I can't cut the tablets up any smaller).

I wonder if I am super-sensitive to the effects of this drug, or if others have found it to be effective at such a low dose. It would be great to hear about the experience of others at this dose level? Thanks.