When it comes to steroids, undoubtedly the side effects that scare people the most are those related to estrogen and its side effects. This is very common when you consider the fact that as a man, the primary reason you have masculine features is due to higher levels of testosterone in comparison to estrogen. For women, the inverse is true. This is very important to understand because whenever you dose with an exogenous compound, you’re going to throw your hormone levels out of whack. This means that whenever you do engage in using one of these compounds, you have to understand that whenever you introduce higher levels of androgenic (testosterone producing compounds) compounds, these are going to result in feedback levels of estrogen in response. This is very important to keep in mind because otherwise, you will be unable to stave off the bad effects that you would otherwise experience from the compound.
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.
Intramuscular testosterone undecanoate is available in the United States , Canada , Europe , and elsewhere in the world.    It is approved in over 100 countries worldwide.   Oral testosterone undecanoate is available in Canada, Europe, Mexico, Asia, and elsewhere but not in the United States.   Intramuscular testosterone undecanoate is marketed most commonly as Nebido in Canada and Europe and as Aveed in the United States while oral testosterone undecanoate is marketed most commonly as Andriol.