> One reason the practice hasn’t been more widespread is that it delivers a diminished dose of a drug. It isn’t clear how much of a high secondary users receive, and some medical experts say there is no more than a placebo effect.

Yes, this makes sense. People are not actually getting high from this, so I doubt that very many people are trying to get high from injecting other people's blood directly.

> Unusual injection practices in Pakistan include selling half-used, blood-infused heroin syringes. (link to https://pubmed.ncbi.nlm.nih.gov/19558668/)

This is the headline, I think. Street vendors sell hits of a syringe that may have already been stuck in someone else. Every time the needle enters a blood vessel, some of the blood goes back into the needle. It's not that users are asking for "blood-infused" syringes, it's that the vendors are sticking multiple people with a single syringe and now 50% of intravenous drug users in Sargodha have HIV.