I have always had allergies and i took allegra (fexofenadine) and it worked for months flawlessly except mental fog, now it won't help with allergies?

Allergies: You should consult an allergist. Try to find out what it is you are allergic to. There are newer drugs in different classes which might help. They work at different sites than antihistamines. Allegra (fexofenadine) is an antihistamine and over time can lose its effectiveness.

Choices exist : Some antihistamines help some people more than others. Zyrtec , Allegra, or Claritin are available as over the counter Rx. Switching may be helpful. There is available by prescription and OTC nasal steroid spray medications that are helpful. There are other medications such as oral antileukotrienes, nasal antihistamine sprays, & combination antihistamine/nasal steroid sprays available. See MD.