Can you take Zyrtec and diphenhydramine?

No, you don't want to take two of the same medications. If the Zyrtec is not taking care of your Allergies, don't take a second antihistamine on top of it. You need to switch from Zyrtec to some other antihistamine. It might be time to see your doctor and try a stronger antihistamine. Taking too much antihistamine can cause breathing difficulty.