Nandrolone phenylpropionate (NPP), or nandrolone phenpropionate, sold under the brand name Durabolin among others, is an androgen and anabolic steroid (AAS) medication which has been used primarily in the treatment of breast cancer and osteoporosis in women. It is given by injection into muscle once every week. Although it was widely used in the past, the drug has mostly been discontinued and hence is now mostly no longer available.

Side effects of NPP include symptoms of masculinization like acne, increased hair growth, voice changes, and increased sexual desire. The drug is a synthetic androgen and anabolic steroid and hence is an agonist of the androgen receptor (AR), the biological target of androgens like testosterone and dihydrotestosterone (DHT). It has strong anabolic effects and weak androgenic effects, which give it a mild side effect profile and make it especially suitable for use in women and children. NPP is a nandrolone ester and a long-lasting prodrug of nandrolone in the body.

NPP was first described in 1957 and was introduced for medical use in 1959. It was the first nandrolone ester to be introduced, followed by nandrolone decanoate in 1962, and has been one of the most widely used nandrolone esters. However, in more recent times, the drug has been largely superseded by nandrolone decanoate, which is longer-acting and more convenient to use. In addition to its medical use, NPP is used to improve physique and performance.