Is diphenhydramine contraindicated in narrow angle glaucoma, acute asthma, and pregnancy? why?

Yes: In patients with closable angles any dilating agent is dangerous as it can precipitate an attach of acute angle closure glaucoma. Anti histamines are contraindicated. Best to see an ophthalmologist and get the angles checked. If they are indeed narrow and closable a laser procedure should be done to prevent a future attack.

Anti-cholinergic : Diphenhydramine's anticholinergic effects are to blame. They cause it to aggravate increased intraocular pressure, and dry secretions in those having asthma attacks, making it hard to clear secretions. Here's a link: http://answers.Yahoo.Com/question/index?Qid=20090328145710aa3jfbq the link is wrong about pregnancy, though. It's category b, not c, for pregnancy and considered safer than other meds.