I find that a single 25mg benadryl usually suppresses the itchiness enough to make it tolerable. Doesn't go away completely, but it definitely helps. Take it like 30 minutes before the heroin. A little bit of itchiness lets me know it's about to really kick in, so I don't mind a little bit. It's when I'm itching like I rolled in a fire ant pile that it pisses me off.