Overview
Testosterone Undecanoate (Injectable) is a testosterone ester used in hormone replacement and performance protocols. As a long-ester formulation, it has a half-life of ~53 days (1272 hours), allowing less frequent dosing. It is tracked in Milligram with one-compartment pharmacokinetic modeling for accurate level predictions.
With a half-life of ~53 days (1272 hours), Testosterone Undecanoate (Injectable) supports less frequent dosing schedules. Administered via IM injection, it has a bioavailability of approximately 95% via its primary route.
Half-life approximately 53 days. Very slow depot absorption via intramuscular administration. Brand names: Nebido, Aveed.
Mechanism of Action
Testosterone Undecanoate (Injectable)'s pharmacological activity involves the following key pathways:
Androgen Receptor Binding
Testosterone Undecanoate (Injectable) binds to androgen receptors in skeletal muscle and other tissues, initiating transcription of androgen-responsive genes involved in protein synthesis and nitrogen retention.
Aromatization
Testosterone Undecanoate (Injectable) is subject to aromatase enzyme conversion, producing estrogenic metabolites. The rate of conversion varies with dose and individual enzyme activity.
Ester Hydrolysis
After injection, the ester bond is cleaved by esterase enzymes, releasing free testosterone into circulation. The ester length determines the rate of release and thus the half-life.
Dosing Protocols
The following protocols represent commonly observed dosing patterns. These are observational summaries, not recommendations.
Dose
750 mg
Route
Intramuscular
Frequency
Every 10-14 days
Duration
8-16 weeks
Dose
1000 mg
Route
Intramuscular
Frequency
Every 10-14 days
Duration
8-16 weeks
Frequently Asked Questions
What is injectable testosterone undecanoate and what are its brand names?
Injectable testosterone undecanoate is a long-chain testosterone ester suspended in castor oil, marketed as Nebido outside the United States and Aveed in the US. It is the longest-acting injectable testosterone formulation available, with a half-life of approximately 20 days. The extended half-life allows for injection intervals of 10–14 weeks once stable levels are established.
How often do injections of testosterone undecanoate need to be administered?
After an initial loading phase – typically a second injection at 6 weeks – maintenance injections are commonly administered every 10 to 14 weeks. The standard Nebido protocol involves an initial injection, a second at week 6, then every 12 weeks thereafter. Aveed follows a similar schedule in a clinical setting.
Why does injectable testosterone undecanoate need to be given in a clinical setting in the US?
In the United States, Aveed is subject to a Risk Evaluation and Mitigation Strategy (REMS) program due to the rare risk of pulmonary oil microembolism – a reaction where oil droplets enter the bloodstream during or after the deep intramuscular injection. This is why the injection must be administered by a healthcare provider, with a 30-minute observation period required afterward.
How long does it take for testosterone undecanoate injections to reach steady state?
Due to the approximately 20-day half-life, full pharmacokinetic steady state is reached after roughly 5 half-lives – approximately 100 days or around 10–14 weeks of consistent dosing. During the loading phase, levels are still rising between doses. This is why a second injection at 6 weeks is commonly used to accelerate the time to therapeutic range.
What volume is injected with testosterone undecanoate and is it painful?
Injectable testosterone undecanoate requires a large injection volume – typically 3 to 4 mL per dose administered as a deep gluteal intramuscular injection. The high volume and viscous castor oil carrier are commonly reported to cause discomfort during and after injection. Slow injection technique is generally noted as important to minimize adverse injection site reactions.
Is injectable testosterone undecanoate suitable for bodybuilding use?
Injectable testosterone undecanoate is primarily observed in medical TRT settings rather than bodybuilding contexts. The very infrequent dosing schedule and fixed high dose (750–1000 mg) make it difficult to titrate for performance purposes. Bodybuilding protocols typically favor shorter-acting esters like enanthate or cypionate that allow for more frequent dosing adjustments.