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TRT · Guide

Testosterone Ester Half-Lives: Cypionate, Enanthate, Propionate & Blends

Testosterone esters differ in how fast their oil depot releases testosterone. This guide sets out what the labels and studies report for each one, where the data run out, and why a blend like "Test 400" has no half-life to report.

PMWritten byPeptide Me Editorial Team12 min read · Updated Sep 2026 · Reviewed against 14 sources
TL;DR
  • Cypionate's FDA label gives a half-life of about 8 days. Intramuscular enanthate estimates run from 2–3 to about 7 days depending on the source, and both have the same labeled dosing interval.
  • Propionate keeps levels up for about two days after an injection. Injectable undecanoate has a half-life of about 34 days in castor oil and is given every 10–14 weeks.
  • Blends such as Sustanon 250 have no single half-life because each ester decays at its own rate. "Test 400" isn't a licensed medicine, and what's in the vial varies by maker.
  • Where labels specify a draw time, it is the end of the interval, after levels have settled. Early or mid-cycle draws catch a level that is still moving.

Testosterone cypionate has a labeled half-life of about eight days after intramuscular injection1. Published figures for intramuscular enanthate range from 2–3 days in one UK product label to about 7 days (172.6 hours) in a 2015 phase II study3,5. Propionate is short-acting: after a single injection, levels stayed above the physiological range for about 48 hours6. Undecanoate is measured in weeks, with a half-life of 33.9 days in castor oil7. "Test 400" has no half-life you can look up. It isn't a licensed medicine, just a label on underground blends, and a mix of several esters has no single half-life anyway. Below, each ester in turn: what the numbers rest on, and what they mean for injection frequency and for when blood gets drawn. For the underlying math of decay and accumulation, start with half-life, explained.

What an ester actually does

Testosterone by itself leaves the blood fast. The US enanthate label notes that reported half-lives of testosterone range from 10 to 100 minutes2. An ester fixes that problem. A side chain is attached at the 17-beta hydroxyl group, which, in the label's words, "increases the duration of action of testosterone," and hydrolysis in the body releases free testosterone again2. The esterified hormone is dissolved in oil and injected into muscle, where it is "absorbed slowly from the lipid phase"1,2. Once it is released, tissue or serum esterases cut off the side chain9,10.

So the half-life of an injectable ester doesn't tell you how fast your body breaks down testosterone. It tells you how fast the oil depot gives up its drug. That's why one molecule can have different numbers depending on the oil, the route and the dose, and why the Nebido label calls its figure a release half-life10.

Why the published numbers disagree

Look up one ester and you may find three different half-lives. That doesn't mean the sources are careless. They usually measure different things:

  1. Modeling method. A population-pharmacokinetic analysis of weekly cypionate injections in 31 healthy men estimated a median half-life of 4.05 days. Assuming no endogenous testosterone, the estimate became 6.87 days. The product label says about eight1,4.
  2. Route. In one phase II trial, subcutaneous enanthate at 100 mg had a half-life of 239.6 hours, against 172.6 hours for 200 mg given intramuscularly3.
  3. Oil vehicle. The same 1,000 mg of undecanoate had a half-life of 20.9 days in tea seed oil and 33.9 days in castor oil7.
  4. Dose. In another undecanoate study, the terminal half-life was 18.3 days at 500 mg and 23.7 days at 1,000 mg8.

Treat any single figure as a planning estimate. The spread between methods can be larger than the difference between cypionate and enanthate themselves.

Ester by ester

Propionate

Propionate is the fast-acting ester. In a Japanese study, men given a single 25 mg intramuscular dose of isotope-labeled propionate kept testosterone from that dose above the physiological level for 48 hours. The ester itself held at 2–4 ng/mL in plasma between 3 and 36 hours after injection6. The study reported duration, not an elimination half-life, and we did not find a current licensed propionate-only product whose label states one. So we don't quote a single number here. The practical point doesn't depend on one: a window of about two days only stays covered with injections every day or two.

Phenylpropionate, isocaproate and decanoate

These three appear mainly as components of Sustanon-type mixtures11. We could not find human pharmacokinetic studies that measured any of them on its own, the way enanthate, cypionate and undecanoate have been studied. The half-life figures that circulate online for these esters usually cite no primary study, so we leave them out rather than repeat them.

Enanthate

Enanthate carries a seven-carbon (oxoheptyl) side chain, as its chemical name shows2. The US generic label describes 200 mg/mL in sesame oil, given intramuscularly at 50–400 mg every 2–4 weeks for hypogonadism, and gives no ester-specific half-life2. A UK enanthate product (250 mg/mL in castor oil) reports a peak of 1,233 ± 484 ng/dL at 24 hours after 200 mg in seven hypogonadal men. Physiological levels (about 500 ng/dL) lasted 11 days, and the label gives a blood half-life of 2–3 days in healthy volunteers5. In men maintained on 200 mg IM enanthate in the 2015 autoinjector trial, the measured half-life was 172.6 hours, about 7.2 days, and the mean time to peak was 33.3 hours3. A subcutaneous enanthate autoinjector, Xyosted, is also FDA-approved and is given once a week into the abdomen13.

Cypionate

Cypionate carries a cyclopentylpropionyl side chain1. Depo-Testosterone comes in 100 and 200 mg/mL strengths in cottonseed oil with benzyl benzoate. Its label states a half-life of "approximately eight days" after intramuscular injection and a dose range of 50–400 mg every 2–4 weeks1. The population model above estimated 4.05 days4. Either way, cypionate sits close to enanthate, which is why the two are usually treated as interchangeable. Our cypionate vs enanthate comparison covers the differences that do exist.

Undecanoate (injectable)

Undecanoate's side chain is undecanoic acid9, and it releases far more slowly. Castor oil is the vehicle in both the US and UK long-acting products9,10, and in castor oil 1,000 mg gave a half-life of 33.9 ± 4.9 days7. The UK Nebido label describes a release half-life of 90 ± 40 days and doses 1,000 mg every 10–14 weeks, with the first interval allowed to shorten to 6 weeks10. In the US, Aveed is 750 mg at initiation, at 4 weeks and then every 10 weeks. Testosterone peaks at a median of 7 days (range 4–42), and steady state comes with the third injection9. Aveed carries a boxed warning for pulmonary oil microembolism and anaphylaxis, requires 30 minutes of observation after each injection and is distributed only through a restricted REMS program9.

EsterLicensed examplePublished half-life / durationLabeled intervalHow solid the figure is
PropionateNone cited hereAbove physiological level for ~48 h after 25 mg IM6—One small 1986 study; duration only, no half-life
Phenylpropionate, isocaproate, decanoateComponents of Sustanon 25011No standalone human data found—Online figures lack primary sources
Enanthate (IM)US generic; UK 250 mg/mL2,52–3 days (UK label)5; 172.6 h ≈ 7.2 days (phase II)3Every 2–4 weeks (US)2Label plus a controlled study; method-dependent
Enanthate (SC)Xyosted13239.6 h ≈ 10 days at 100 mg3Weekly13Phase II study plus FDA label
Cypionate (IM)Depo-Testosterone1~8 days (label)1; 4.05 days (population model)4Every 2–4 weeks1Label plus a modeling study
Undecanoate (IM)Aveed (US), Nebido (UK)9,1033.9 days in castor oil7; release half-life 90 ± 40 days (Nebido label)1010 weeks after loading (Aveed); 10–14 weeks (Nebido)9,10Several studies plus two labels
Sustanon 250 (four-ester blend)Sustanon 250 (UK)11No single half-life; peak at 24–48 h, back to the lower normal limit in ~21 days11Usually every 3 weeks11Label data for the mixture as a whole
"Test 400"None; not a licensed productUnknown; composition varies by maker—No published data
◑ Limited human dataEach figure above comes from a product label or a published pharmacokinetic study, and each describes a specific population, dose and formulation. They are reference points for reading a curve, not a prediction of your own levels. Only bloodwork measures those.

Blends: Sustanon, "Test 400" and why a mix has no single half-life

A blend puts short- and long-acting esters in one oil. The licensed example is Sustanon 250. Each 1 mL contains 30 mg propionate, 60 mg phenylpropionate, 60 mg isocaproate and 100 mg decanoate in arachis (peanut) oil, 176 mg of actual testosterone in all11. Its label describes how the mixture behaves as a whole. Total testosterone peaks at about 70 nmol/L roughly 24–48 hours after a dose and returns to the lower limit of the normal male range in about 21 days. One 1 mL injection every three weeks is usually adequate11. Because of the arachis oil, Sustanon is contraindicated in people allergic to peanuts or soya11.

Notice what the Sustanon label leaves out: a half-life. There's a reason for that. Each ester in the vial releases at its own rate, so the level in blood is several decay curves added together. In the first days the fast esters dominate and the curve falls steeply. Weeks later, most of what remains comes from the slowest ester and the curve flattens. How long the level takes to fall by half depends on when you start counting, so a single number for a blend is wrong for most of the dosing interval. That is also why our half-life calculator has no blend preset: it models one exponential curve, and a blend isn't one.

What "Test 400" is

The short answer: "Test 400" isn't a licensed medicine. None of the FDA-labeled testosterone injectables we reviewed (cypionate, enanthate, subcutaneous enanthate and undecanoate) is sold under that name1,2,9,13. No product by that name appears among the testosterone medicines listed in the UK's electronic medicines compendium, which covers enanthate, Sustanon 250 and undecanoate injections14. The name refers to a strength, 400 mg/mL, used by underground manufacturers, and the esters and ratios in the vial change from one maker to the next. With no fixed composition there is no half-life to report, and we found no published pharmacokinetic study of any product sold as "Test 400".

The label on these vials may also not match what's inside. A 2022 systematic review covered 19 studies and 5,413 black-market anabolic steroid samples. It estimated that 36% were counterfeit and another 37% were substandard. Samples were found with no active ingredient, the wrong one, a different amount than labeled, or a different set of ingredients12. One example in the review was a product sold as a four-ester testosterone mix that contained only propionate12. For a blend, even the ester list printed on the vial can't be relied on, so any half-life quoted for "Test 400" is a guess on top of a guess.

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What half-life means for injection frequency

The labeled intervals for cypionate and enanthate are 2–4 weeks1,2. With a half-life of roughly a week, that interval covers two to four half-lives, so levels fall a long way from peak to trough. The UK enanthate data show the shape: a peak at 24 hours, then physiological levels for about 11 days5. Splitting the same weekly amount into more frequent, smaller injections narrows that swing. Our TRT microdosing guide covers the reasoning and the trade-offs, and half-life, explained shows how the ratio of interval to half-life sets the peak-to-trough ratio.

Undecanoate works the other way round. Its release is slow enough that injections are spaced 10 weeks or more apart. Loading doses (the second Aveed injection at 4 weeks, or a Nebido interval shortened to 6 weeks) get levels into range sooner than waiting several full intervals would9,10. Propionate's two-day window6 sits at the other extreme.

Timing bloodwork around the ester

Every injectable ester produces a curve that peaks and then declines, so the time of the draw changes the result. Where the labels give a draw time, they put it at the end of the interval:

  1. Subcutaneous enanthate (Xyosted): a trough measured 7 days after the most recent dose, 6 weeks after starting and 6 weeks after any dose change. The target range is 350–650 ng/dL13.
  2. Undecanoate (Nebido): measure "at the end of an injection interval." Levels should fall in the lower third of the normal range, and results below normal point to a shorter interval10.
  3. Intramuscular cypionate and enanthate: the US labels we reviewed don't specify a draw time1,2. Pick a consistent point in the cycle, usually just before the next injection, and keep it the same for every draw so results can be compared.

Levels also need several half-lives to settle after a start or a change, so an early draw measures a moving target. Aveed reaches steady state at the third injection, 14 weeks in9, and Nebido between the third and fifth10. Blends add another complication: a day-7 trough and a day-14 trough reflect different mixes of esters. The full panel and the timing rules are in our TRT bloodwork guide.

Milligrams of ester vs milligrams of testosterone

A milligram of ester contains less than a milligram of testosterone, because the side chain accounts for part of the weight. The UK enanthate label equates 250 mg of enanthate with about 180 mg of testosterone5. Sustanon 250 delivers 176 mg of testosterone per 250 mg of mixed esters11, and a 1,000 mg Nebido vial corresponds to 631.5 mg10. Cypionate (molecular weight 412.61) and enanthate (400.59) differ by only about 3%1,13.

Using the half-life calculator with these esters

The presets in our half-life calculator use the figures in this guide. Test-C is 192 hours, the eight-day label value1. Test-E is 173 hours (172.6 rounded) for intramuscular injection, and Test-E SC is 240 hours (239.6 rounded) for the subcutaneous autoinjector, both from the same trial3. Also try the lower cypionate estimate of 4.05 days, about 97 hours4, and compare the two curves to see how much the choice of source changes the picture. Undecanoate falls outside the calculator's range, which stops at a 240-hour half-life and a weekly interval. Blends have no preset, for the reasons above.

A calculator draws a model. In the Peptide Me app, the same engine runs on the doses you actually log and can be calibrated against your lab results, which gets closer to your real curve than any table can.

The bottom line

Cypionate and enanthate both have half-lives of about a week, and the gap between them is smaller than the spread in published estimates for either one. Propionate lasts about two days, and undecanoate lasts weeks. A licensed blend like Sustanon has label data for the mixture but no single half-life. "Test 400" has neither a licensed composition nor a measured half-life, and independent testing of black-market steroids shows how often the vial's label is wrong.

⚠︎Educational information only — not medical advice. Testosterone is a prescription medication; discuss any route or dose change with a licensed clinician.

FAQ

There isn't one. "Test 400" isn't a licensed medicine: it is a 400 mg/mL label used by underground manufacturers for blends whose esters and ratios vary from maker to maker. Even a well-defined blend has no single half-life, since each ester releases at its own rate. A 2022 review of black-market steroid testing also estimated that 36% of samples were counterfeit and a further 37% substandard.
The Depo-Testosterone label gives about eight days after intramuscular injection. A population-pharmacokinetic study of weekly injections estimated 4.05 days, or 6.87 days when endogenous testosterone was assumed to be zero. The spread comes from the method, not the drug.
By most estimates slightly, but the ranges overlap. Published figures for intramuscular enanthate run from 2–3 days (one UK label) to about 7.2 days (a 2015 trial). Cypionate figures run from about 4 to 8 days depending on the source. The US labels give both the same dose range and interval.
The label doesn't give one, because Sustanon 250 is a mix of four esters. It reports that testosterone peaks at about 70 nmol/L 24–48 hours after injection and returns to the lower limit of the normal range in about 21 days. One injection every three weeks is usually adequate.
In a study of a single 25 mg intramuscular dose, testosterone from the injection stayed above the physiological level for about 48 hours. That short window is why propionate is associated with injections every day or two.
Where labels specify a time, it is the end of the interval: 7 days after a dose for subcutaneous enanthate (Xyosted) and the end of the injection interval for undecanoate (Nebido). For IM cypionate or enanthate, draw at the same point in the cycle every time, usually right before the next injection, and only once levels have settled.

References

Primary sources — PubMed / NEJM / The Lancet.

  1. DailyMed. DEPO-TESTOSTERONE (testosterone cypionate) injection, solution — Prescribing Information. U.S. National Library of Medicine. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=cfbb53d4-b868-4a28-8436-f9112eb01c39
  2. DailyMed. TESTOSTERONE ENANTHATE injection, solution (Eugia US LLC) — Prescribing Information. U.S. National Library of Medicine. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=82a98132-9d5f-40a5-8c4f-f52f2a5de60e
  3. Kaminetsky J, Jaffe JS, Swerdloff RS. Pharmacokinetic Profile of Subcutaneous Testosterone Enanthate Delivered via a Novel, Prefilled Single-Use Autoinjector: A Phase II Study. Sex Med. 2015;3(4):269-279. https://pmc.ncbi.nlm.nih.gov/articles/PMC4721027/
  4. Bi Y, Perry PJ, Ellerby M, Murry DJ. Population Pharmacokinetic/Pharmacodynamic Modeling of Depot Testosterone Cypionate in Healthy Male Subjects. CPT Pharmacometrics Syst Pharmacol. 2018;7(4):259-268. https://pmc.ncbi.nlm.nih.gov/articles/PMC5915615/
  5. electronic medicines compendium (emc). Testosterone Enantate 250 mg/ml Solution for Injection Ampoules — Summary of Product Characteristics. Alliance Pharmaceuticals Ltd. https://www.medicines.org.uk/emc/product/3733/smpc
  6. Fujioka M, Shinohara Y, Baba S, Irie M, Inoue K. Pharmacokinetic properties of testosterone propionate in normal men. J Clin Endocrinol Metab. 1986;63(6):1361-1364. https://pubmed.ncbi.nlm.nih.gov/3782423/
  7. Behre HM, Abshagen K, Oettel M, Hübler D, Nieschlag E. Intramuscular injection of testosterone undecanoate for the treatment of male hypogonadism: phase I studies. Eur J Endocrinol. 1999;140(5):414-419. https://pubmed.ncbi.nlm.nih.gov/10229906/
  8. Zhang GY, Gu YQ, Wang XH, Cui YG, Bremner WJ. A pharmacokinetic study of injectable testosterone undecanoate in hypogonadal men. J Androl. 1998;19(6):761-768. https://pubmed.ncbi.nlm.nih.gov/9876028/
  9. DailyMed. AVEED (testosterone undecanoate) injection — Prescribing Information. U.S. National Library of Medicine. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=f80f025b-17d8-40af-8739-20ce07902045
  10. electronic medicines compendium (emc). Nebido 1000 mg/4 ml solution for injection — Summary of Product Characteristics. Grünenthal Ltd. https://www.medicines.org.uk/emc/product/14631/smpc
  11. electronic medicines compendium (emc). Sustanon 250, 250mg/ml solution for injection — Summary of Product Characteristics. Aspen Pharma Trading Ltd. https://www.medicines.org.uk/emc/product/5373/smpc
  12. Magnolini R, Falcato L, Cremonesi A, Schori D, Bruggmann P. Fake anabolic androgenic steroids on the black market – a systematic review and meta-analysis on qualitative and quantitative analytical results found within the literature. BMC Public Health. 2022;22:1371. https://pmc.ncbi.nlm.nih.gov/articles/PMC9288681/
  13. DailyMed. XYOSTED (testosterone enanthate) injection — Prescribing Information. U.S. National Library of Medicine. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=8a3d204c-be26-49e0-8599-0ac12a272e81
  14. electronic medicines compendium (emc). Search results for "testosterone" (UK licensed products), accessed September 2026. https://www.medicines.org.uk/emc/search?q=testosterone

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