Testosterone undecanoate is a long-chain fatty acid ester derivative of the natural androgen testosterone. It is a white or almost white crystalline powder that is practically insoluble in water but soluble in organic solvents such as ethanol and oils. This esterification significantly enhances the lipophilicity of the parent hormone. Its primary and well-defined application is in medicine as an active pharmaceutical ingredient (API). It is used in androgen replacement therapy for the treatment of conditions associated with testosterone deficiency in men, such as hypogonadism. Due to its lipophilic nature, it is formulated in oil-based injectable solutions or oral capsules (in combination with specific absorption enhancers), allowing for sustained release and prolonged pharmacological action compared to unesterified testosterone. As a prodrug, testosterone undecanoate is hydrolyzed in the body to release free testosterone, which then exerts its effects by binding to androgen receptors. It is also listed on the World Anti-Doping Agency's prohibited list due to its performance-enhancing potential. Handling requires standard precautions for potent hormonal substances.