Is it possible to get "rebound" side effects from stopping an antihistamine? i've been taking one for 5 months, and have sinus problems since stopping

You probably need it: Rebound to cessation of antihistamines would be unusual, . If you took a combination antihistamine/decongestant, it would be more common. However, you had been taking an antihistamine for 5 months. Had you taken it for a nasal problem? My hunch is that your nasal/sinus problem, for which you had taken the antihistamine in the first place, recurred the moment you stopped it.