My son is on cephalexin 500 mg, 3 times a day due to cellulitis. His cellulitis started with what seemed to be a small patch of poison ivy, but grew huge. He has been on cephalexin for 24 hours now and his cellulitis has grown (the doctor said we should see a difference in 48 hours). He is very uncomfortable, says it burns and itches. Can I give him diphenhydramine hydrochloride to help with the itching.