My otherwise VERY vigorous and healthy 80 year old husband (takes NO meds practically ever and is physically very active) has sinus congestion and upper respiratory cough with copious post nasal drip. This follows a very sore throat that is better this evening. Can he take Benadryl (diphenhydramine 25 mg) and Robitussin DM at the same time. That's all we have in the arsenal.......