Testosterone enanthate ( USAN , BAN ) (brand names Delatestryl , Testostroval , Testro LA , Andro LA , Durathate , Everone , Testrin , Andropository ), or testosterone heptanoate , is an androgen and anabolic steroid and a testosterone ester .    Along with testosterone cypionate and testosterone propionate , it is one of the most widely used testosterone esters.  Testosterone enanthate was first introduced in 1952.  Administered via intramuscular injection , it is the most widely used form of testosterone in androgen replacement therapy . 
The partition coefficient of the ester in question is important because is effects how long the drug itself stays in the system. If the testosterone transfers too quickly from the oil to the blood, the result is a sudden spike in testosterone which then rapidly drops once the dose has been used up. In the example of free testosterone injected into the muscle from a water suspension (as in Aquiviron, mentioned above), the testosterone is essentially immediately available to the bloodstream due to its low partition coefficient, and thus there is an immediate spike of testosterone which is used up quickly in the body.
In androgen-responsive target tissues such as the skin, scalp, and prostate, the high relative androgenicity of testosterone is dependant on its reduction to dihydrotestosterone (DHT). The 5-alpha reductase enzyme is responsible for this metabolism of testosterone. The concurrent use of a 5-alpha reductase inhibitor such as finasteride or dutasteride will interfere with site-specific potentiation of testosterone action, lowering the tendency of testosterone drugs to produce androgenic side effects. It is important to remember that both anabolic and androgenic effects are mediated via the cytosolic androgen receptor. Complete separation of testosterone’s anabolic and androgenic properties is not possible, even with total 5-alpha reductase inhibition.