The loss of functioning veins (venous sclerosis) is a root cause of suffering for long-term heroin injectors. In addition to perpetual frustration and loss of pleasure/esteem, venous sclerosis leads to myriad medical consequences including skin infections, for example, abscess, and possibly elevated HIV/HCV risks due to injection into larger jugular and femoral veins. The etiology of venous sclerosis is unknown and users' perceptions of cause/meaning unexplored. This commentary stems from our hypothesis that venous sclerosis is causally related to heroin acidity, which varies by heroin source-form and preparation. We report pilot study data on first ever in vivo measurements of heroin pH and as well as qualitative data on users' concerns and perceptions regarding the caustic nature of heroin and its effects. Heroin pH testing in natural settings is feasible and a useful tool for further research. Our preliminary findings, for example, that different heroin source-forms and preparations have a two log difference in acidity, have potentially broad, vital and readily implementable harm reduction implications.