 [ Symptoms ] i have a very swollen and painful finger i cannot move any of the joints [ Disease ] they are both antihistamines the zyrtec is purported to be a non sedating antihistamine and diphenhydramine benadryl is a sedating antihistamine when used as a sleep aid it can interfere with deep restful dream like states so personally i am not a big fan of using it if he wants to use both then just take the zyrtec in the morning the diphenhydramine wears off in about 4 hours so it will be gone