Choosing a Carrier Oil

Most legal compounding pharmacies use castor, sesame, or sunflower oil.

Most DIY brewers use MCT oil (AKA Viscoleo). MCT is more viscous and so absorption rate is thought to be quicker. However, its viscousness makes it easier to inject.

We use MCT oil and feel that the trade-off is worth it. It may be absorbed faster by the body, but we like how easy it is to inject. Also, when combined with the long half-life of EEn or TC it doesn’t seem to cause issues for us.

Lena says:

“MCT oil… made from a mix of C8 (caprylic) and C10 (capric) fatty acids (not C8 only)”

This is like the MCT oil listed here, which is also listed as pharmaceutical grade. We haven’t researched the C8/C10 thing, but we will say that you should definitely get pharmaceutical grade if you can.

Some more info from an article on Transfeminine Science:

It is in any case known from other studies that different oil vehicles are absorbed at different rates from the injection site (Svendsen & Aaes‐Jørgensen, 1979; Schultz et al., 1998; Larsen et al., 2001) and can result in different concentration–time curve shapes (Ballard, 1978 [Excerpt]; Knudsen, Hansen, & Larsen, 1985). This is thought to be due to differences in oil lipophilicity and depot release rates. Viscosity of oils has also been hypothesized to potentially influence rate of depot escape (Schug, Donath, & Blume, 2012). However, research so far has not supported this hypothesis (Larsen & Larsen, 2009; Larsen, Thing, & Larsen, 2012). Oil vehicles can vary with injectable estradiol preparations even for the same estradiol ester. For instance, pharmaceutical estradiol valerate is formulated in sesame oil, castor oil, or sunflower oil depending on the preparation (Table). It is notable however that these three oils have similar lipophilicities (Table). On the other hand, homebrewed injectable estradiol preparations used by DIY transfeminine people often employ medium-chain triglyceride (MCT) oil as the oil vehicle. This oil (in the proprietary form of Viscoleo) has notably been found to be much more rapidly absorbed than conventional oils like sesame oil and castor oil in animals (Svendsen & Aaes‐Jørgensen, 1979; Schultz et al., 1998; Larsen et al., 2001). In addition, although based on very limited data, MCT oil has been found to give spikier and shorter-lasting depot injectable curves in humans (Knudsen, Hansen, & Larsen, 1985). As such, injectable estradiol preparations using MCT oil as the vehicle may have differing and less favorable concentration–time curve shapes than pharmaceutical injectable estradiol products. Other excipients, like benzyl alcohol, as well as factors like injection site and volume, have additionally been found to influence pharmacokinetic properties with depot injectables (Minto et al., 1997; Kalicharan, Schot, & Vromans, 2016). Excipients besides oil vehicle also vary by formulation (Table).