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Hi Dr Stan here to help you. Hives are caused by an allergic reaction to something ingested usually by eating a food that you are allergic to. Hives can also be caused by something you put on your skin like cologne or soap or even detergent used to wash your clothes. Benadryl can cause urinary retention which is the "bad" reaction you probably had. The best way to avoid hives is to stay away from known allergens. I hope this is helpful and I am here to answer any additional questions you may have. I am compensated for the information provided only after you give me a positive rating. Thank you for allowing me to be of help today.