Testosterone phenylbutyrate, also known as testosterone phenylbutanoate, testosterone 17ò-phenylbutyrate, and androst-4-en-17ò-ol-3-one 17ò-phenylbutyrate, is a synthetic, injected anabolic-androgenic steroid (AAS) and an androgen ester â specifically, the C17ò phenylbutyrate (phenylbutanoate) ester of testosterone â which was never marketed. It is a prodrug of testosterone and, when administered via intramuscular injection, is associated with a long-lasting depot effect and extended duration of action.