Sustainbolic injection is a trade name for an oil-based injectable blend of four esterized testosterone compounds:

30mg Testosterone Propionate 60mg Testosterone Phenylpropionate 60mg Testosterone Isocaproate 100mg Testosterone Decanoate

The different testosterone esters in Sustainbolic injection provide for different half lives. Esterization of the testosterone molecules provides for a sustained (but non-linear) release of testosterone from the injection depot into the blood plasma.

While the intention of the mixed testosterone esters in Sustainbolic injection is to provide more stable serum testosterone levels, a single long-ester testosterone, such as testosterone cypionate or testosterone enanthate, may actually provide more stable serum testosterone levels. It is the preferred method of testosterone replacement in the UK as detailed in the British National Formulary.