Just an update on something I’ve been working on.
I’ve been experimenting with ways to extend the release of testosterone undecanoate beyond what you typically get from standard castor oil formulations—even the ones I’ve brewed myself. The idea was to take what already works and push it further by looking at how pharmaceutical-grade long-acting injectables (like Nebido) are designed, then going deeper into the science behind controlled-release systems.
While researching this, I kept seeing patterns in studies where compounds like aluminum stearate were being used to turn oil-based solutions into gel-like depots after injection. These depots form in situ.. meaning they stay injectable as a liquid but then solidify in the muscle once injected, slowing down the release of the active ingredient. This is the same general principle behind how some long-acting contraceptives and anti-psychotic injectables work. One of the best papers I came across that broke this down was from Wilkinson et al. in the European Journal of Pharmaceutical Sciences (2022), which outlined how lipid-based oleogels using aluminum stearate could be used to extend the release profile of lipophilic drugs.
From there, I dug into veterinary formulations and depot technologies used in other long-acting drugs and found that 2–5% aluminum stearate in castor oil can actually hold up really well. Once injected intramuscularly, the stearate forms a 3D gel matrix as it cools and settles in the muscle. The testosterone undecanoate is then released slowly through diffusion and ester hydrolysis over time. The literature suggests that you can realistically get 35–50 days of stable release, depending on the injection volume and site.
For example, Bounoure et al. (2017) did long-term depot studies using similar aluminum stearate systems and confirmed extended residence and controlled release. I also looked at Nebido’s own data, which reports a ~34-day half-life with a 1000 mg/4 mL injection of TU in plain castor oil.. no gelator used. So theoretically, if you introduce even a mild gel structure on top of what castor oil already does, you should be able to extend the release without needing NMP or co-solvents like BB or EO.
I’m putting together a simple formulation that’s injectable when warm but gels once in the muscle. It’s built off my existing 370 mg/mL TU in castor oil solution. By just adding around 3% aluminum stearate, I can push it into oleogel territory without increasing injection volume. At 2 mL, that gives me 740 mg per shot, which... based on the literature.. should cover a full 9-week interval. Or longer! The data would have to be extracted from blood work and long term adherence to this... So hey this is likely something I can update with once I start my experiment.
