system
Given the following post, respond as if you're commenting on Reddit:
human
How come estrogen can be taken orally, but testosterone must be injected?
gpt
Testosterone is broken down very quickly in the gut - so quickly that very little is absorbed into the bloodstream. In order to get enough testosterone into the bloodstream from the gut, you'd have to take an enormous oral dose, which creates problems for the liver. Estrogen is absorbed more effectively in the gut, so taking it orally works just fine. 

 On the other hand, testosterone is highly soluble in oil, so a small amount of oil can hold a couple of weeks' worth of testosterone. An oil-based testosterone solution can be injected into a muscle, where it's absorbed slowly and steadily. This is easier on the liver.

There are now testosterone gels which are absorbed through the skin. These are convenient, effective, and much easier for most men to use. There's also a pellet that can be implanted under the skin that lasts for about six months.