There are a number of factors that can affect the potency of a particular drug compound. One such factor, and perhaps one of the most important, is the half-life of the agent. The term half-life refers to the duration it takes for half of a given drug dosage to break down in the body, it is not half of the total activity time, as this figure always refers to the time it takes to metabolise 50% of what is still in the body. For example, injecting 100mg steroid with a half-life of 4 hours, at the four-hour mark we should have only 50mg left as active. After another 4 hours have passed the drug is still in the body, however another half-life has expired and the total active dosage will be around 25mg. It may take several half-lives before the drug is completely inactive. [nbcite author="R. A. S. Hemat" title="Andropathy" year="2007" pages="330" type="book" ]

Half-life is not an easy reference for a total time a drug will be found active in the body, but more a guide to optimising a dosing schedule and avoiding unwanted peaks and througs. Natural steroid hormones have very short half-lives, which can make mantaining a normal blood level very difficult. For example, the half-life of free testosterone in the blood is only a few minutes, and from the site of injection it is well short of one hour. It is also so easily processed by the liver.

The farmaceutical industry has developed and ultimately adopted methods for extending steroid half-life. 17 Alkylation is a process in which an extra carbon atom is added to the steroid molecule at the 17th position. This atom accupies a bond needed for the steroid to reduce (to inactive) 17-keto form, totally inhibiting this pathway of metabolism. The addition of the 17 alkylation works to extend the half-life of the steroid considerably. With it half-lives measured in hours instead of only minutes.

Unfortunately, 17 alkylation also can lessen the ability of the steroid to bind to the androgen receptor (AR). This alteration is the most favorable for oral dosing. Since the liven cannot process this type of steroid well, a large percentage will make it to the blood stream intact. However, it is also somewhat toxic to the liver, and therefore less than ideal.

Most injectable steroid compounds utilise esters to increase their half-lives in the body. Esterification is a process where a carboxylic (fatty) acid is attached to the steroid molecule at the 17th beta position. One purpose of this is to protect its active 17-hydroxyl group. It is a prime target of steroid metabolism, and with the ester present this is prevented. The ester also makes the steroid compound more oil soluble. This makes it more difficult for the blood to pick it up and carry it into circulation, and slows the rate the drug can leave the injection site. As a result, an inactive deposit of steroid can sit at the site of injection, releasing slowly for days or weeks into the blood stream. Once free in the blood the ester is removed quickly by enzymes, and the base steroid is rendered active.

The half-life of injectable compounds occurs in two ways:

1. The first is the half-life for the release of the steroid from the injection site. This is usually measured in days with most commercial steroid preparations. In fact the total active lifespan of most oil-based esterified injectables is measured in weeks, sometimes several weeks.
2. The second measure is its half-life in circulation. This is more a figure for personal interest sake than any practical application, however, as the only relevant  measure to the user is its release half-life.

[nbcite print="default" ]