I have unisom at home. when my allergies flare up, couldnt i take a unisom, since its an antihistamine?

You could, but don't: Unisom contains the active ingredient Diphenhydramine that is in benadryl (diphenhydramine). While the desired effect of decreasing allergy symptoms will happen, side effects that can be limiting are: drowsiness, delayed reaction time, impaired cognition (thinking), etc. If you have a job where you need to think, be alert, drive, operate machinery, etc, it's best to take claritin, allegra, zyrtec, clarinex, xyzal.