Anything you do every day multiple times a day you end up getting good at. However junkies have famously poor health and do not generally practice harm reduction. As such, needles get reused, sites are not properly cleaned, tourniquets are rarely used, veins get abused, missed, burst, or the user may simply find themselves too dehydrated, shakey, irritable, or others unable to successfully hit a vein. Some may spend hours unsuccessfully stabbing themselves over and over.

Those who do the bare minimum will generally always use fresh needles, rotate inject sites, and are usually able to use either hand to find various veins in both arms.