I think theres different levels to this fetish.
I am a gay man and I have experience using meth IV with various people and the part I like is knowing that we are feeeling good and horny and high. I knew one person who wanted me to mix up a shot, shoot up half of it then he got off on shooting the rest of the mix with my blood in it. That was really fucked up an after I became freaked out he found someone else to indulge his fantasy with.
Another guy wanted me to shoot him up in his cock which I never did, then finally the last time I saw him I thought fuck it im never going to see him again and did it. This guy was not a great human. He wateched a 19yo guy drink 19mls of GBL and die in his home. I often worried that if something happened to me he would not do what was needed to keep me alive.
One time I blew out on GBL(became unconscious but still breathing) and woke up with a towel over me while he continued playing with the other guy there.
I get off on the fact that I know they are high and horny now, its nothing to do with actually shooting up or blood or sharing needles which ive found in some people and that is instantly a red flag for me. Also shooting up other people is a red flag for me, I will never, EVER do that again its so dangerous and people lie. I always had a rule that I wouldnt assist anyone who had never IV'd before but people lie and I gave someone their first shot, we went through around 60-70k worth of drugs in around a year and I got blamed for stealing all of his money and wrecking his life.
Truth to a meth user is often subjective.