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Muscle & performance

Testosterone Undecanoate

The ultra-long testosterone ester - very infrequent injections, classic in TRT.

1 min read 10 sources

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Glass vial with golden oil and a black TPD label on a white podium, next to it a large blue 3D steroid molecule with a long ester chain in a bright lab
Approved · in the US since 2014

Summary

At a glance

Sources checked on 26 Oct 2026

Per year instead of 17 or more
4 injections

Long-term study with 12-week intervals, enanthate was given every 3 weeks for comparison3

Trough level after 9 weeks
16.3 nmol/L

Mean before the next injection, with enanthate it always stayed below 10 nmol/L3

Largest study
1,045 men

Phase 3 on male hormonal contraception, monthly injections over 30 months6

Oil embolism in 90,092 doses
28 reports

Below 0.1 percent per injection, mostly resolved without medical help7

Phase 3 in the registry
8 trials

injection only, 3 of them phase 2/3, as of 26 Oct 20269

  • Testosterone Undecanoate is testosterone with a very long undecanoate ester. It is released extremely slowly from the oil depot in the muscle and acts on the androgen receptor.102
  • In a long-term study four injections per year were enough to keep trough levels in the normal range. With enanthate it took 17 or more.34
  • In an animal model the peaks stayed much smaller than with enanthate. The level stayed slightly raised for 45 days and in the normal range for another 56 days, with enanthate it reached the lower limit after 31 days.8
  • In the largest study with 1045 men the monthly injection suppressed sperm production in most participants. After stopping it recovered in all but two.6
  • A rare peculiarity is oil embolism in the lung right after the injection. The depot injection has been approved in the US since 2014. Everything here is the state of research, not a recommendation.710
Profile

Updated June 2026

Class
Testosterone ester

ultra-long

Use
Intramuscular

subQ possible, very infrequent

Half-life
very long

weeks

WADA
banned

anabolic

Sizes
3 sizes

300mg/ml, 250mg/ml, 40mg*100tabs

Contents 13 sections
  1. At a glance
  2. What is Testosterone Undecanoate?
  3. How it works
  4. Use and dosing
  5. What the research shows
  6. Research status
  7. Side effects
  8. Storage
  9. PCT / after coming off
  10. Related compounds
  11. FAQ
  12. Glossary
  13. Sources

What is Testosterone Undecanoate?

Testosterone Undecanoate is testosterone with the ultra-long undecanoate ester. In clinical TRT (brand name Nebido) it is injected only every few weeks - the compound is released extremely slowly. That gives very stable levels without frequent injecting.

This information is for research and educational purposes only. No medical advice.

Comparison

Short and long esters

The same compound at the same receptor with esters of different length, as stated in the TPD lexicon. Tap a card to open its entry.

Short ester Testosterone Propionate AR Medium ester Testosterone Phenylpropionate AR Long ester Testosterone Enanthate AR Long ester Testosterone Cypionate AR Very long ester Testosterone Decanoate AR
Ultra-long ester Testosterone Undecanoate AR

How it works

Like all testosterone esters: androgen receptor activation, protein synthesis, nitrogen retention, partial aromatization to estradiol. The only difference is the very slow release due to the long ester.

Where it acts

One receptor, several organs

Tap a point or a card. Grey = effect seen in trials.

  1. Hypothalamus and pituitary

    Gonadotropins LH and FSH suppressed5

    Observed in trials
  2. Testes

    Sperm production suppressed, recovered in almost all after stopping6

    Observed in trials
  3. Blood

    Haemoglobin and haematocrit rose in the first 30 weeks, then no further4

    Observed in trials
  4. Liver and blood lipids

    LDL cholesterol fell, HDL fell at first and rose again with long-term use4

    Observed in trials
  5. Lung

    Rarely oil embolism right after the injection, mostly resolved within minutes7

    Observed in trials
  6. Skeletal muscle

    Grip strength improved only after about 90 weeks4

    Observed in trials

Targets in the ChEMBL database10

Compound CHEMBL2107067: one recorded mechanism, agonist with direct binding to the human receptor.

Androgen receptor
Gene
AR
UniProt
P10275
ChEMBL
CHEMBL1871
Action
Agonist, direct binding
Blue 3D molecules leave a glass droplet one by one and sink onto bright, blue glowing muscle fibres
The long ester releases testosterone only slowly from the depot. Symbolic image.

Use and dosing

Dosing

Dosing, titration and syringe

Loading the dosing information …

  • Getting started and stepping up, drawn as a staircase
  • Units on the U-100 syringe for every step
  • How long one vial lasts
Glass vial with golden oil and a black TPD label, a disposable syringe in front, a white desk calendar in the background
Very long ester: few injections with long intervals. Symbolic image.

What the research shows

Testosterone research (Bhasin et al., NEJM 1996) applies to all esters equally. Testosterone undecanoate is clinically established as a TRT preparation; AAS pharmacology: Kicman 2008.

Study results

What the trials measured

Interactive: switch tabs, tap points, show or hide arms. Arms are named relative to each other, no dosing advice.

Testosterone before the next injection3

Mean trough level in nmol/L, 40 men with hypogonadism, long-term study

After the first two injections3 6-week interval
Undecanoate 14.1 nmol/l
Week 303 after a 9-week interval
Undecanoate 16.3 nmol/l

With enanthate every 3 weeks the mean trough levels before each injection always stayed below 10 nmol/L. In the follow-up with 12-week intervals they stayed between 14.9 and 16.5 nmol/L, estradiol and DHT rose in parallel and stayed in the normal range.

Sperm production in the largest study6

1045 men, monthly injection, phase 3

Not suppressed enough6 within the first 6 months
Undecanoate 4.8%
Rebound during use6 after suppression
Undecanoate 1.3%
Overall method failure6
Undecanoate 6.1%
Pregnancies per 100 men6 over 24 months
Undecanoate 1.1%

855 men entered the efficacy phase, 733 completed it. No serious adverse events were reported, sperm production returned to the normal range in all but two. Research data, not a contraceptive.

Hourglass with white sand on a bright lab bench between two racks of sample tubes
Long-term studies over months and years. Symbolic image.

Research status

Where the research stands

Shown are registered trials with Testosterone Undecanoate as an injection. Trials with the oral capsule are not listed. As of 26 Oct 2026. 9

  1. Preclinical Release in an animal model (monkeys), compared with enanthate
  2. Phase 1 no registered trial
  3. Phase 2 7 trials, 4 completed
  4. Phase 3 8 trials, 8 completed
  5. Approval in the US since 2014
816 participants in phase 3 Approved: ChEMBL lists Testosterone Undecanoate with phase 4 as the highest phase and first approval in 2014, the year the depot injection was approved in the US.10
Show all 15 phase 2 and 3 trials on a timelineHide timeline 12 completed 3 running 0 with results
Phase 2
Phase 3

Bar: start to primary endpoint (planned for running trials). Green = completed, blue = progress until 26 Oct 2026.

Side effects

  • Estrogen-related: water retention, gyno risk with high aromatization.
  • Possible hematocrit rise.
  • HPTA suppression - PCT topic.
  • Androgenic: acne, hair loss in predisposed individuals.

Storage

Store the oil solution at room temperature, protected from light - no fridge needed. Out of reach of children.

Four glass vials with golden oil on a white shelf, next to them a folded light blue cloth
Oil solution at room temperature and protected from light, no fridge needed. Symbolic image.

Related

Related compounds

Same receptors: all in the TPD lexicon.

Testosterone Undecanoate
Testosterone Enanthate Long ester, every 1 to 2 weeks Testosterone Cypionate Long ester, close to enanthate Testosterone Decanoate Very long ester, closest relative Testosterone Phenylpropionate Medium-length ester Testosterone Propionate Short ester, frequent doses Test Suspension No ester, lasts only hours

FAQ

Common questions about Testosterone Undecanoate

Short answers, each backed by the sources below.

How long does one injection last?

Very long. In a long-term study trough levels with 12-week intervals stayed between 14.9 and 16.5 nmol/L and thus in the normal range. In an animal model (monkeys) the level stayed slightly raised for 45 days after one injection and in the normal range for another 56 days, with enanthate it reached the lower limit after 31 days.38

Is Testosterone Undecanoate approved?

Yes. The ChEMBL database lists Testosterone Undecanoate as an approved drug with first approval in 2014. That year, on 5 March 2014, the depot injection was approved in the US. Trials are still running on the registry, for example on multiple sclerosis and on pain with opioid use. Everything here is the state of research, not a recommendation.1079

How does it differ from enanthate?

The compound is the same, only the ester is longer. In a randomised study with 40 men undecanoate every 12 weeks was at least as effective and safe as enanthate every 3 weeks. With enanthate the level fell below 10 nmol/L before each injection, with undecanoate it stayed above.43

What is an oil embolism in the lung?

If small oil droplets reach the bloodstream during the injection, they can travel to the lung. A post-approval review in the US found 28 reports among 90,092 distributed doses over 4.3 years, below 0.1 percent per injection. 21 of 22 cases resolved, most within 30 minutes, more than 60 percent without medical help. In the serious cases the injection technique often played a role.7

What happens to the body's own hormone production and fertility?

Testosterone from outside suppresses the hormone axis, LH and FSH are turned down. In the largest study with 1045 men sperm production was not suppressed in 4.8 percent, in 1.3 percent the sperm count rose again during use. After stopping, sperm production returned to the normal range in all but two participants.56

Which side effects were seen?

In a study with 13 men there were no serious side effects. Body weight, haemoglobin, haematocrit, prostate volume and PSA rose slightly. In a study over 30 weeks plus a 114-week follow-up haemoglobin and haematocrit rose in the first 30 weeks and then no further, without exceeding the upper limit. LDL cholesterol fell, HDL fell at first and later rose again.54

Does the muscle research on testosterone apply here too?

In principle yes, because once the ester is split off the same testosterone acts. The best-known study used enanthate though: with strength training healthy men gained 6.1 kg of fat-free mass in 10 weeks. With undecanoate grip strength improved in a long-term study only after about 90 weeks.142

Glossary

Terms explained briefly

Dotted words in the text open these explanations directly.

Androgen receptor
Protein inside the cell that testosterone binds to. Genes are then read, for example for muscle building.
Aromatization
Conversion of part of the testosterone into estradiol by the enzyme aromatase.
Depot
The oil solution stays in the muscle after the injection and releases the compound over weeks.
Ester
An attached fatty acid chain. The longer it is, the more slowly testosterone is released from the oil depot. It is split off in the blood.
Haematocrit
Share of red blood cells in the blood. If it rises a lot, the blood gets thicker.
Hormone axis (HPTA)
Hypothalamus, pituitary and testes together control the body's own testosterone production. Testosterone from outside suppresses this axis.
Intramuscular
Into the muscle, for example the glute or thigh.
LH and FSH
Pituitary hormones. LH drives testosterone production in the testes, FSH drives sperm production.
Oil embolism (POME)
Small oil droplets reach the bloodstream during the injection and travel to the lung (pulmonary oil microembolism).
Trial phases
Phase 1 tests tolerability in a few people, phase 2 looks for the dose, phase 3 confirms benefit and safety in many. Phase 4 runs after approval.
Trough level
The lowest value just before the next injection.

Practice

PCT / after coming off

Anabolic substances suppress the body's own testosterone production in the research context. Anyone who comes off a cycle (rather than running it continuously) typically uses a post-cycle therapy (PCT) to restart the natural axis (HPTA).

The standard scheme in the research/literature context combines a SERM (tamoxifen and/or clomifene, alternatively enclomiphene) over roughly four weeks, optionally preceded by HCG to re-sensitise the testes before the SERM block. An aromatase inhibitor (anastrozole/exemestane) belongs more to on-cycle estrogen control than to the PCT itself.

Tip: order the PCT items together with the cycle so they are already on hand when you come off.

This information serves research and educational purposes only. Not medical advice.

Tamoxifen (Nolvadex) SERM, core of PCT View Clomid (Clomifen) SERM, core of PCT View Enclomiphene (Androxal) SERM alternative View HCG testicular restart before PCT View Arimidex (Anastrozole) aromatase inhibitor, on-cycle View Aromasin (Exemestane) aromatase inhibitor, on-cycle View

Evidence

Sources

Show all 10 sourcesHide sources 8 Studies1 Trial registry1 Database checked 26 Oct 2026

Studies

  1. Supraphysiologic testosterone doses, muscle size and strength in healthy men

    Bhasin et al., New England Journal of Medicine, 1996

    DOI 10.1056/NEJM199607043350101 PMID 8637535
    Checked 26 Oct 2026 via doi.org, Crossref, PubMed
  2. Pharmacology of anabolic steroids, review

    Kicman, British Journal of Pharmacology, 2008

    DOI 10.1038/bjp.2008.165 PMID 18500378
    Checked 26 Oct 2026 via doi.org, Crossref, PubMed
  3. Levels during long-term use, compared with enanthate

    Schubert et al., Journal of Clinical Endocrinology and Metabolism, 2004

    DOI 10.1210/jc.2004-0897 PMID 15531493
    Checked 26 Oct 2026 via doi.org, Crossref, PubMed
  4. Efficacy and safety with a 114-week follow-up, compared with enanthate

    Minnemann et al., Journal of Endocrinological Investigation, 2008

    DOI 10.1007/BF03346421 PMID 18852533
    Checked 26 Oct 2026 via doi.org, Crossref, PubMed
  5. Repeated injections in hypogonadism, 24 weeks

    Nieschlag et al., Clinical Endocrinology, 1999

    DOI 10.1046/j.1365-2265.1999.00881.x PMID 10619981
    Checked 26 Oct 2026 via doi.org, Crossref, PubMed
  6. Phase 3 male contraception study with 1045 participants

    Gu et al., Journal of Clinical Endocrinology and Metabolism, 2009

    DOI 10.1210/jc.2008-1846 PMID 19293262
    Checked 26 Oct 2026 via doi.org, Crossref, PubMed
  7. Oil embolism in the lung after approval, review of 4.3 years

    Pastuszak et al., Sexual Medicine, 2020

    DOI 10.1016/j.esxm.2020.01.009 PMID 32184081
    Checked 26 Oct 2026 via doi.org, Crossref, PubMed
  8. Release in an animal model compared with enanthate

    Partsch et al., European Journal of Endocrinology, 1995

    DOI 10.1530/eje.0.1320514 PMID 7711892
    Checked 26 Oct 2026 via doi.org, Crossref, PubMed

Trial registry

  1. All trials on ClinicalTrials.gov

    ClinicalTrials.gov, U.S. National Library of Medicine

    Checked 26 Oct 2026 via ClinicalTrials.gov API v2

Database

  1. Mechanism and approval in ChEMBL

    ChEMBL, EMBL-EBI (CHEMBL2107067)

    Checked 26 Oct 2026 via ChEMBL

In the shop

Testosterone Undecanoate in the catalog

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