What would work better for motion sickness once vomiting has started... iv benadryl or iv phenergan (promethazine)?

First : Once vomiting started nothing can be given orally, just IV or intra muscular. Both drugs are histamine blockers bit have a relaxing effects too. I would try with benadryl (diphenhydramine). If in i would dilute it in saline and inject very slowly. Hope it will help. Did you try scopolamine patches before being exposed to motions?

Motion sickness: Both can work all right. Have not seen a lot of patients on i/v Benadryl lately, but has seen Phenergan (promethazine) used more frequently through this route.