I do not understand why you would want to add another antihistamine to your Diphenhydramine/opiate cocktail? If that's not doing it, why not just up the dose of the opiate? Adding another antihistamine is just going to make the effects of the antihistamines (delirium) more prominent which most people don't find pleasant. If you want to know other antihistamines that you could experiment with instead of the diph. Then Hydroxyzine and promethazine are two that come off the top of my head, I like hydroxyzine, it's got some good anxiolytic properties for being non-benzodiazepine and non-GABAergic in general. It's also more potent than benadryl, with less of the weird side effects.