Hmm, antiemetics work on the H1 receptor as well, so they should be able to block the opioid nausea well enough. In short, opioids release histamine → histamine stimulates balance/motion pathways in the brainstem → causes nausea and dizziness. Both Antihistamines and antiemetics should be able to prevent that. Technically, you can take both if you want. Shouldn't be a problem