Any anti-histimine can do the job but the one I've personally had the most success with, as with what I've seen with other people, is Benadryl or mock-benadryl (I actually get the Generic Target Brand Benadryl because it's the cheapest by far. You get like 200 for less then $5, whereas brand name is like 10 for $5).
If it isn't working I'd say you aren't using enough... you have to considerably increase your dosage of benadryl as you can get a tolerance to it and if you are doing high levels of opiates. Doesn't matter if you can handle 3 grams of heroin IV, the more dope in your system the more anti-histimine you need to combat it's histimine release.
When you take upwards of 6 you get pretty drowsy with Benadryl though, so keep that in mind if you take high doses of opiates and you feel you need more then 6. Means you may need a different antihistimine (though no clue what would be better then benadryl)
I mean how many are you taking? I usually take 4-6.