What happens to the test carrier / ester oil in your body when you inject into the muscle. How is it metabolised/ where does the oil go?

Testosterone in its true form, like most drugs, comes in a powder. Most drugs are hydrophilic, so if they need to be injected, they are mixed with saline, bacteriostatic water, or sterile water and then injected.

Steroids (sex hormones like testosterone) are typically lipophilic and hydrophobic. In other words, they dissolve in and are “attracted to” lipids (e.g., fats), and they are poorly soluble in and repelled by water. This is why carrier oils are used instead of saline to inject.

When an oil solution of a sex steroid is administered by intramuscular or subcutaneous injection, the solution is trapped within the tissue compartment it is injected into and remains there. As tissue is essentially a water mixture, the oil solution stays together inside the tissue compartment and does not easily separate or distribute. Instead of rapidly dissolving, the fats and sex steroids at the edges of the oil solution are very slowly absorbed into the surrounding water in the tissues. Once they have escaped the oil depot into the surrounding tissue fluid, they can be distributed into the bloodstream. The oil is used as energy and the testosterone attaches to androgen receptors to enable its effects. Eventually, the whole oil solution will be absorbed. This process can take several days.