Diphenhydramine HCl comes in 25 mg and 50 mg strengths (Benadryl is a common brand). For adults and teens, the diphenhydramine dosage is 25 to 50 mg every 4 to 6 hours. The maximum amount you should take in one day is 300 mg. Keep in mind, taking higher doses can increase the risk of side effects, including drowsiness.