What is more effective on poison ivy. allegra or benadryl (diphenhydramine)?

Either : You can take either. Both would improve itchiness somewhat but not treating the inflammation due to contact dermatitis to your suspected poison ivy. Benadryl (diphenhydramine) acts faster but it does not last as long. Plus it makes you drowsy. Allegra last longer and does not cause drowsy. You might need to see a doctor if symptoms not controlled. Corticosteroids treat the inflammation and hasten the recovery.

Benadryl (diphenhydramine): For the control of itching, Benadryl (diphenhydramine) should be more effective but may make you more sleepy. If it is a severe case, and there is no contraindication, steroids will give you the best relief in the shortest amount of time.