My Husband Has Been Having Reoccurring Lip Swelling For The

My husband has been having reoccurring lip swelling for the last few months sometimes one a week or it will skip a week. We haven't change foods, drinks, or diet or even soaps. We can't fig out what is causing them Benadryl seems to help but it takes hours for the swelling to go down. We took him er first time and they didn't say what the cause was just gave him a steroid and Benadryl. ?????