after I cook it till it boils
A little warming is ok but its counterproductive to let your heroin/water solution boil. Not only are you loosing potency due to the heat(212°F/100°C is too hot) degrading it, hence, why caffeine(higher boiling point) is sometimes added for smoking, you are also introducing cuts/fillers/whatevers that would otherwise NOT become a component of your solution. HIGH QUALITY tar, whether its crunchy, shiny, glass-like rocks or gooey, sticky, taffy-like gum, needs absolutely no heat to dissolve. Just a little stirring or a couple minutes of waiting.
From what I understood, the tar is usually more likely to cause abscesses than #3 or #4. It's not safe at all to shoot a thick goo
When i was in the depths of my 10yr opiate/opioid dependency i spent a handful of those years living in a region that ONLY had BTH. In that time i had a 2.5-3yr period of shooting every day, multiple times a day and i never once got an abscess. Even with missing(or partially missing), on average, a few times a week.
Also, once you filter it, you SHOULD have a beautifully clear(tinted red-ish, gold or brown-ish), NOT cloudy, goopy or highly viscous solution regardless if it was super low quality(in which case you might have to filter 2x) starting out like sludge.
The acidity of BTH seems most likely to be the culprit of veinous damage in long-term shooters if they(we) are serious, practicing harm reductionists.
I've never heard of heroin being cut with sugar before. Maybe because we have only brown heroin where I live.
Heroin is often cut with lactose, mannitol or other sugars.