A rash is not a normal symptom. It is an indication that something is happening. It could be a sensitivity to something in your environment. If so, taking any kind of medicine would only mask it or provide temporary relief. To get rid of the rash permanently, you would need to eliminate the trigger from your environment. With skin rashes, it is often something like a lotion, soap, or perfume. So that's something to look at. Once the trigger is removed, your rash -- if it is allergic in nature -- should resolve quickly.

But, more importantly, has your "rash" truly been diagnosed as an allergy? I ask because skin conditions can sometimes be caused by other things than an allergy. So I would strongly encourage you to see your doctor, if you haven't already, to eliminate anything more serious than a contact allergy.