Does your hayfever antihistimine work for you better if you alternate brands every month?

Probably not. There have been many studies on the good effects of new generation antihistamines and up to this point, these medications have not been shown to develop "tolerance." in these clinical trials, the medications continued to work just as well with continued use. This is group data. However, very long term studies are not available. For a specific individual it's possible that a change may help.