That isn't a legitimate scenario. They were asking the drug dealer(s) to cut their heroin with the drug. Now an untrained individual is responsible for mixing in the proper dosage for their clients (for a drug that has not been proven to be safe/work, and maybe certain ratios dont dissolve quickly enough, or maybe smoking the heroin causes the drug to under go a molecular transformation from the heat), which is completely uncontrollable anyways because everyone shoots w/ different amounts and frequencies? AND assuming the dealer cares enough to notify each of their clients? You think that is ethical?

To answer your direct question though, yes, I am comfortable telling heroin users they cannot have access to HIV retardants until they have been proven safe, because you've made the false claim that this drug is actually an HIV retardant, when in reality it is untested.