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Rash over lips and genitals once or twice an year for the past 3 years. This could be most probably due to Fixed Drug Eruption
, a type of drug rash
. Whenever you are exposed to certain drugs for which you are allergic, it will produce lesion at the same part of body preferably on lips and genitals where the tissue are loose and thin.
Each time exposed to the same drug the rash may be same or even slightly larger area and after the challenged drug stopped it will heal and the swelling and itching came back to normal.
So you have to find the offending drug and avoid the same totally will relieve your problem.