I think you misunderstood the study in Pakistan. > The injector injects 3 ml and keeps 2 ml (the scale) as injection fee. Heroin users will first inject the d

I think you misunderstood the study in Pakistan. > The injector injects 3 ml and keeps 2 ml (the scale) as injection fee. Heroin users will first inject the drug, then immediately draw blood back into the syringe and then reinject that, to accomplish a kind of mixing the drug with the blood in the syringe. The study claims that people leave some of that blood/drug mix in the syringe as a fee.