Benadryl, or diphenhydramine hydrochloride, when taken by mouth, works within 15 to 60 minutes. When injected through a muscle, it takes 20 to 30 minutes to work. When injected through a vein, it works rapidly. The effects of diphenhydramine hydrochloride work for approximately 4 to 8 hours, regardless of the form in which it is taken. Any comments?