An esterified form of the base steroid Testosterone, much like Enanthate, Cypionate and Sustanon 250.
It's a Superlipophillic, oil-based injectable that slows the release of the steroid into the blood stream.
But compared to Enanthate and Cypionate, Propionate is a very short ester and is still released quite fast. As such more frequent injections are needed. Levels will peak after 24-36 hours and begin tapering from there on out, making the longest possible time-span between injections, at least or proper results, about 3 days. Most athletes will opt to inject 50-100 mg every day to every other day.