Testosterone Undecanoate (Castor Oil)
Nebido · TU · Reandron
Very long-acting ester (Nebido). Dosed 1000 mg every 10–14 weeks for TRT. NHS standard.
Key takeaways
- Longest-acting testosterone ester — ~34-day half-life, dosed 1000 mg every 10–14 weeks.
- Only 4–5 injections per year, but slow onset and minimal dose-tuning flexibility.
- Serum peak ~10 days post-injection; levels run flatter than with short esters.
- Same monitoring as any TRT — estradiol, hematocrit, PSA, lipids; suppression is complete.
Overview
What it is
Testosterone undecanoate in castor oil (Nebido, Reandron) is the longest-acting testosterone ester, built for deep intramuscular depot injection. The 11-carbon ester chain and the viscous castor-oil carrier slow hydrolysis so testosterone releases gradually over months. It is a standard TRT option in Europe and on the NHS.
Pharmacokinetics
The half-life is about 34 days, with a serum peak roughly 10 days after injection and Cmax near 800 ng/dL at the clinical 1000 mg dose. That profile supports 1000 mg every 10–14 weeks, which keeps most men in range with only 4–5 injections per year.
Harm reduction
It aromatizes to estradiol and suppresses the HPTA like any testosterone, so estradiol, hematocrit, PSA, and lipids still need monitoring. The trade-off for that convenience is slow onset and almost no dose-tuning flexibility, so it suits stable maintenance TRT rather than rapid titration. It is not hepatotoxic.
Reported dosing protocols
Protocols reported in research literature and clinic/community use — informational context, not a dosing recommendation.
| Use case | Dose | Route | Frequency | Duration |
|---|---|---|---|---|
| TRT (label) | 1000 mg (Nebido) / 750 mg (Aveed, US) | IM deep gluteal | Every 10–14 weeks | Ongoing |
Pharmacokinetic summary
- Model tier
- Advanced (t½ + Cmax + Tmax + F)
- Half-life
- 34 days
- Cmax
- 800 ng/dL
- Tmax
- 10.0 days
- Bioavailability
- 65%
- Typical dose
- 750–1000 mg
Harm reduction
- Aromatizes
- Yes
- DHT derivative
- No
- Injection frequency
- Every 10–14 weeks
Frequently asked questions
Why is undecanoate dosed only every few months?
The long ester plus castor-oil carrier gives a ~34-day half-life, so one 1000 mg depot holds levels in range for 10–14 weeks. A loading injection at week 4 builds steady state faster; everything after that is maintenance.
Is Nebido available in the US?
Not under that brand. The US equivalent is Aveed (testosterone undecanoate 750 mg in castor oil), which sits behind a REMS program because of rare post-injection pulmonary oil microembolism. Nebido is the standard in Europe and the UK.
Undecanoate vs enanthate for TRT — which is better?
Neither is inherently better. Undecanoate means far fewer injections and flatter levels but slow titration and a long tail if problems arise. Enanthate allows quick adjustment at the cost of weekly injections. The choice usually tracks prescriber and country.
References
- Bayer Nebido Prescribing Information — Cmax calibrated to ng/dL serum peak (Nebido 1000mg → ~600–900 ng/dL).
- Bayer Nebido Prescribing Information — Label dosing schedule and PK parameters.
Related articles
Related compounds
Testosterone Enanthate
Long-acting testosterone ester; the most common TRT and cycle base. Reaches steady state in ~5 weeks.
Testosterone Cypionate
Standard US TRT ester with a slightly longer release tail than enanthate. Steady state ~4–5 weeks.
Testosterone Propionate
Short-acting ester with sharp peaks; requires frequent injection. Useful for frontloading.
Protocols featuring Testosterone Undecanoate (Castor Oil)
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