This is good for my purposes. It means high doses can be taken for brain effects without disturbing stomach acidity. The question remains, of course, whether coinciding CNS tolerance develops.

H1 antihistamines enhance the opioid high in humans. Hospitals sometimes administer antihistamines in combination with opioids. It's not hard to find people online who are using this combination recreationally.

It is interesting to note that they report potentiation of 10mg but not 5mg of morphine. Thus, it is possible that antihistamine potentiation is dependent on dose of opioid, or that antihistamines raise the analgesic ceiling. That is to say, antihistamines may raise the maximum possible effect from opioids without raising the effects of lower doses much. However, antihistamine combination did tend to enhance the effects of lower dose morphine, but the effect was not as dramatic (therefore not reaching statistical significance in a study of this power).