^As kids, I don't think anyone would imagine putting a needle in their arms period.
But the truth of the matter is, that people have used antihistamine's along side opiates for a long time (promethazine/codeine, Dipipanone/cyclizine etc..) However, as someone that unfortunately used to shoot buprenorphine mixed with diphenhydramine, I would recommend that anyone who absolutely must try IV opiate/antihistamine to use a micron filter, because vein damage occurs very quickly with Benadryl, and I've read that with some antihistamine's such as promethazine, have black boxed warning's issued by the FDA on injectable ampules due to the risk of "sever tissue injury associated with the administration of this drug."