Cos codeine releases more histamine than heroin probably. Morphine's bad for it too. Some opies are heavy on the histamine whilst others barely an itch.
Curious: True enough but most folk do get serious itchy issues on high (or even low for that matter) doses of codeine. It's infamous for the itchiez. It's not a tolerance thing either cos I was getting through 3-4g per day in (1g per dose) and even after many, many months (or at times in the past even longer and probably at higher dose) I'd always get the most insane itchiez. Actually got so used to it I genuinely thought it was part of the opiate buzz rather than just a massive - and potentially quite iffy at those levels - histamine release. Got used to it but never got any less itchy