Boldenone undecylenate, or boldenone undecenoate, sold under the brand names Equipoise and Parenabol among others, is an androgen and anabolic steroid (AAS) medication which is used in veterinary medicine, mainly in horses. It was formerly used in humans as well. It is given by injection into muscle.

Side effects of boldenone undecylenate 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 moderate androgenic effects, weak estrogenic effects, and no risk of liver damage. Boldenone undecylenate is an androgen ester and a long-lasting prodrug of boldenone in the body.

Boldenone undecylenate was introduced for medical use in the 1960s. In addition to its medical use, boldenone undecylenate is used to improve physique and performance.