Heroin and Itchiness

My friends called me out for itching like a heroin addict from a rash I have, but none of could figure out why heroin addicts itch. So can anyone explain the mechanism of why taking heroin or withdrawing from heroin leads to being itchy? Thanks!

According to this site opiates such as heroin, morphine, etc release histamine, hence the itching and perspiration of junkies.