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.
Testosterone, like many anabolic steroids, was classified as a controlled substance in 1991. Testosterone is administered parenterally in normal and delayed-release (depot) forms. In September 1995, the FDA approved testosterone transdermal patches (Androderm), and many transdermal forms and brands are now available including implants, gels, and topical solutions. A testosterone buccal system, Striant, was FDA-approved in July 2003; Striant is a mucoadhesive product that adheres to the buccal mucosa and provides a controlled and sustained release of testosterone. In May 2014, the FDA approved an intranasal gel formulation of testosterone (Natesto). A transdermal patch (Intrinsa) for hormone replacement in women is under investigation; the daily dosages used in women are much lower than for products used in males. The FDA refused approval for Intrinsa in 2004 stating that more data regarding safety, especially in relation to cardiovascular and breast health, were required.