Anabolic Steroids/Hormones
Testosterone Undecanoate (Castor Oil)
Also called Nebido · TU · Reandron
Long-acting testosterone undecanoate is used for TRT when fewer injection appointments are a priority. Nebido and Aveed can last many weeks between injections, but their doses and starting schedules differ.
Dosage & duration
Two different castor-oil products for adult testosterone replacement.
Nebido · adult testosterone replacement
Product label- Dose
- 1000 mg per injection (4 mL)
- How often
- Into muscle every 10–14 weeks; the first interval may be shortened to 6 weeks
- How long
- Ongoing replacement with periodic review
The prescriber selects the interval using symptoms and testosterone measured near the end of the interval.
Sources: 1
Aveed · US adult testosterone replacement
Product label- Dose
- 750 mg per injection (3 mL)
- How often
- Into muscle initially, again at 4 weeks, then every 10 weeks
- How long
- Ongoing replacement; no fixed cycle
Administered by a healthcare professional; 30-minute observation after every injection because of serious injection reactions.
Sources: 2
These schedules do not apply to oral testosterone undecanoate or unverified MCT-oil products.
Long-acting TRT
Long intervals, slower adjustments
Everyday convenience
-
Fewer injection days
Long intervals reduce how often an injection needs to fit into everyday life.
What comes with it
-
Slow to adjust
Once injected, the long-lasting depot cannot be stopped like a daily gel.
Nebido and Aveed differ They have different injection amounts and starting schedules; follow the instructions for the actual product.
Why choose a long injection?
The appeal is having fewer injections to fit around work, travel and everyday life. Some people value going months without thinking about a shot; others report tiredness or lower libido before the next appointment. Neither experience describes everyone. This is replacement for diagnosed deficiency, not a special muscle-building hormone.
What if symptoms return before the next shot?
Bring the timing of those symptoms and the date of the last injection to the prescriber. Nebido’s product information uses symptoms together with testosterone measured near the end of an interval to guide treatment. Another person’s experience or a smooth plot cannot determine someone else’s appointment schedule.
Why two different schedules?
Nebido and Aveed contain the same ester in castor oil, but they are distinct approved products with different injection amounts and starting instructions. The first shorter interval is part of establishing treatment; it is not the permanent repeat interval.
Side effects and follow-up
Testosterone can affect fertility, blood counts, blood pressure and prostate monitoring. With oil injections, sudden cough, chest symptoms or breathing difficulty during or after administration need immediate attention. Aveed’s supervised observation requirement remains relevant even after previously uneventful injections.
Comparing the plot
Use the curve to explore overlap across long intervals. It cannot determine the best appointment date or predict a blood test. Do not transfer the castor-oil schedule to a differently formulated MCT product.
Model details: Testosterone Undecanoate (Castor Oil) · relative timing
Compare timing and buildup for this compound. The values are not blood concentrations or a measure of strength.
Shows a relative timing scenario; absolute blood concentrations are not calibrated.
Timing assumptions and sources
33.9 day terminal from Behre 1999; 7 day Tmax from current Nebido label is total testosterone. Combining these yields only a relative illustrative shape.
One 1 mg input peaks at 1 relative unit after a single dose. Other amounts scale proportionally; overlapping doses add when accumulation is on. Different compounds use separate relative scales.
- Timing half-life used
- 813.6 hours
- Absorption half-life assumed
- 35.5809 hours
- Reference amount for the relative scale
- 1 mg
- The faster rate is inferred to match the 168 h source-scoped peak time; absorption half-life 35.580899063328104 h is a mathematical shape parameter, not separately measured.
- A single 1-unit input has a peak of 1 relative unit; scaling and repeated-dose addition are mathematical assumptions. This normalization is independent of the example dose.
- The 14-person 1999 formulation and current Nebido label describe different kinetic summaries; 33.9 day terminal and 90 day release half-life are not interchangeable.
- No source-matched baseline/suppression model was established here. Use relative output while retaining a stated castor-oil timing scenario.
- Stored 0.65 resembles an ester mass fraction, not a demonstrated systemically available fraction.
- Behre 1999 phaseI testosterone undecanoate
1000 mg castor-oil IM group had 33.9±4.9 day terminal half-life; tea-seed group 20.9±6.0 days. No MCT group.
- Nebido 1000 mg/4 mL SmPC §§2, 5.2
1000 mg ester corresponds to 631.5 mg testosterone; first-injection total serum testosterone peaks at 38 nmol/L after 7 days; reported release half-life 90±40 days.
testosterone-undecanoate-castor-relative-timing-2026-10 · version 1
Safety and interaction notes Read the limitations and discuss clinical questions with a professional
- Aromatizes
- Yes
- DHT derivative
- No
Common questions
Is testosterone undecanoate taken for a cycle?
Medical replacement is usually ongoing while indicated; it is reassessed rather than assigned a bodybuilding cycle.
Can I use the same schedule for Aveed and Nebido?
No. Their label amounts and starting schedules differ. Match the exact product and prescribed plan.
Sources Labels, studies and notes behind this guide
Catalog evidence: Other human research. This describes the type of evidence, not a clinical review of every claim.
Summary and fact source checks: 2026-10-07. This records source checking, not medical review of the whole page.
Notes on the quick facts
- Main benefit
- Useful for people who prefer fewer treatment days.
- Trade-off
- The long depot cannot be removed like stopping a daily gel.
- Product matters
- Do not combine the starting interval from one with the dose of the other.
- 1. Nebido — UK product information
1000 mg/4 mL castor-oil IM preparation; maintenance every 10–14 weeks and first interval can shorten to six weeks, guided by levels and symptoms.
- 2. Aveed — US prescribing information
750 mg/3 mL castor-oil IM preparation: initial, four weeks, then every ten weeks. Healthcare administration and 30-minute post-injection observation; POME/anaphylaxis warnings.
- 3. Behre 1999 phaseI testosterone undecanoate
1000 mg castor-oil IM group had 33.9±4.9 day terminal half-life; tea-seed group 20.9±6.0 days. No MCT group.
- 4. Nebido 1000 mg/4 mL SmPC §§2, 5.2
1000 mg ester corresponds to 631.5 mg testosterone; first-injection total serum testosterone peaks at 38 nmol/L after 7 days; reported release half-life 90±40 days.
- 5. FDA — testosterone product information
Current regulator context; blood-pressure monitoring and product-specific labeling should take precedence over older boxed-warning summaries.
Keep reading
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