Does benadryl (diphenhydramine) help with nausea and headache?

Not usually: Benadryl (diphenhydramine) is fine for allergies, and if seasonal issues trigger headaches, may be of limited benefit. But if you have migraine, will not help, and you are far better to try excedrin migraine or aleve. However, depending on your severity and frequency, you may want to consult your doctor and try to prevent the headaches.