Can i take robitussin nighttime cough dm 5 hours after taking benadryl?

Robitussin Nighttime Cough DM contains a cough suppressant (dextromethorphan) and an antihistamine (doxylamine succinate). Benadryl (diphenhydramine) is another antihistamine. Not sure why you would want to take 2 antihistamines, as they both do the same thing. In addition, together, they can cause lowering of your breathing rate which can be dangerous.