the first to use was arnold
synthol (or syntherol) first showed up in the early 1990s, but bodybuilders were using site injections long before that. certain anabolic steroids cause inflammation for a few days, so injecting a cc or two into a small muscle like the bicep or delt may cause an increase in size, but only for a few days.
with synthol, the increase in size is due to the volume of oil. with site injections, the increase in size is due to your body's reaction (inflammation) to the irritating substance.