Sustanon 250 contains a mixture of four testosterone compounds, which are modified with the addition of carboxylic acid esters (propionic, propionic phenyl ester, isocaproic, and decanoic acids) at the 17-beta hydroxyl group. Esterified forms of testosterone are less polar free testosterone, and are absorbed more slowly from the area of injection. Once in the bloodstream, the ester is removed to yield free (active) testosterone. Esterified forms of testosterone are designed to prolong the window of therapeutic effect following administration, allowing for a less frequent injection schedule compared to injections of free (unesterified) steroid. Sustanon 250 is designed to provide a rapid peak in testosterone levels (24-48 hours after injection), and maintain physiological concentrations for approximately 21 days. Sustanon 250 is designed to provide a fast yet extended release of testosterone and is highly favored by athletes to promote strong increases in muscle mass and strength. It is usually incorporated into bulking phases of training, when added water retention will be of little consequence, users are generally more concerned with raw mass then definition. Some do incorporate it into cutting cycles as well, but typically in lower doses and usually accompanied by an aromatase inhibitor to keep estrogen levels under control. Sustanon 250 is effective on its own but many do stack it with other anabolic/androgenic steroids for a stronger effect.