Fusion Testosterone Enanthate is a modified form of testosterone, where a carboxylic acid ester (enanthoic acid) has been attached to the 17-beta hydroxyl group. Esterfied 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. The half-life of testosterone enanthate is approximately 8 days after injection.