Nandrolone phenylpropionate is a modified form of nandrolone, where a carboxylic acid ester (propionic phenyl ester) has been attached to the 17-beta hydroxyl group. Esterified steroids are less polar than free steroids, and are absorbed more slowly from the area of injection. Once in the bloodstream, the ester is removed to yield free (active) nandrolone. Esterified steroids 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. Nandrolone phenylpropionate provides a sharp spike in nandrolone release 24-48 hours following deep intramuscular injection, and declines to near baseline levels within a week.