Diphenhydramine is the active ingredient there. Same as most OTC sleeping pills.
Anywho, make sure it's not counterindicated by any other medications you might be taking.
Also, do you have any other concomitant indications? Sneezing, watery eyes, runny nose, swelling, itching, hives, ...
There are other types of allergic reactions that a simple antihistiminergic-class drug won't address.
I'm no doctor, and this isn't medical advice in any way, or any kind of endorsement. If you think you have an allergic reaction to something, you should stop using it and consult with your general practitioner who should probably refer you to a specialist...