Can i give my 5 year old benadryl (diphenhydramine) and dextromethorphan polistirex cough suppressant?

Medicine: You can give him both medications but expect him to be very sleepy, in other words, be careful in how you treat your child.

Bad idea: Both are sedatives and can reduce breathing and cause serious problems for your child. Don't suppress the cough, treat the cause. Bring him to the pediatrician.

No Or Need Monitor: Data demonstrate that the specified agents may interact with each other in a clinically significant manner. The benefits of concomitant use of these two medications usually outweigh the risks. An appropriate monitoring plan should be implemented to identify potential negative effects. Dosage adjustments of one or both agents may be needed in a minority of patients.

Probably OK: Aside from the additive drowsiness effect, i see no problem with the combination if your child really needs them.