Testosterone isocaproate is testosterone with a short, branched six-carbon ester (isocaproate, also called 4-methylpentanoate) attached. Like other testosterone esters, it is injected into muscle in oil, released slowly from the depot, and then cut back to ordinary testosterone in the blood.
It is not sold as a medicine on its own in the drug registers we checked. Its only medical use is as one of four esters in mixed-ester products such as Sustanon 250 and Omnadren 250, each of which contains 60 mg of it per mL. The single most important fact: no human pharmacokinetic or clinical study has tested testosterone isocaproate by itself. Everything known about it in people comes from studies of these blends.
This page summarizes what product labels, drug registers, blend studies and anti-doping research report. It is for education only and is not medical advice.
| Property | Detail |
|---|---|
| Drug class | Androgen; testosterone ester (anabolic-androgenic steroid) |
| Products | Component of Sustanon 250 (UK) and Omnadren 250 (Poland), 60 mg/mL in each; no single-ester licensed product found |
| Route | Deep intramuscular injection, as part of an oil solution |
| Chemistry | Testosterone 17β-isocaproate, C₂₅H₃₈O₃, 386.6 g/mol |
| Testosterone content | ≈75% of the ester’s weight (60 mg ester ≈ 45 mg testosterone) |
| Half-life | No published human value for the isolated ester; the Sustanon 250 blend peaks at 24–48 h and falls to the low-normal range in ≈21 days (label) |
| US status | Not in any FDA-approved product; testosterone and its esters are Schedule III |
| WADA status | Prohibited at all times (S1.1 anabolic androgenic steroids, including esters) |
What it is
The ester is made from isocaproic acid (4-methylpentanoic acid), a six-carbon fatty acid with a branch near its end. It is attached at the 17β position of testosterone. PubChem lists the compound as testosterone isocaproate, CAS number 15262-86-9, with the formula C₂₅H₃₈O₃ and a molecular weight of 386.6 g/mol (PubChem).
The ester has no hormonal role of its own. It makes the molecule more fat-soluble, so it leaves the oil depot more slowly. Once in the blood, esterase enzymes remove it and release testosterone (UK SmPC). A review of steroid pharmacology explains that the duration of action depends on the length of the acid chain and on the formulation. In general, the longer the chain, the more slowly the ester is released (Kicman, Br J Pharmacol 2008).
This places isocaproate between the short and the long esters. The table compares it with sibling esters, using PubChem molecular weights and PubChem’s computed lipophilicity score (XLogP3, higher means more fat-soluble):
| Ester | Acid chain | Molecular weight | Share that is testosterone | Computed XLogP3 |
|---|---|---|---|---|
| Propionate | 3 carbons | 344.5 g/mol | ≈84% | 4.4 |
| Isocaproate | 6 carbons (branched) | 386.6 g/mol | ≈75% | 5.5 |
| Phenylpropionate | 3 carbons plus a phenyl ring | 420.6 g/mol | ≈69% | 5.8 |
| Enanthate | 7 carbons | 400.6 g/mol | ≈72% | 6.3 |
| Cypionate | 8 carbons, with a ring | 412.6 g/mol | ≈70% | 6.4 |
| Decanoate | 10 carbons | 442.7 g/mol | ≈65% | 8.0 |
Testosterone itself weighs 288.4 g/mol. The share is 288.4 divided by the ester’s molecular weight.
Lipophilicity is only a rough guide to release rate. It suggests isocaproate is released faster than enanthate or decanoate and slower than propionate, but no human study has measured its release rate directly.
About quoted half-lives. Online sources give specific half-lives for testosterone isocaproate, often in the range of several days. We could not trace any of these figures to a human pharmacokinetic study of the isolated ester, so we do not repeat them here. The only human timing data come from the four-ester blend, described below.
Products that contain it
Testosterone isocaproate appears in medicine only as part of four-ester blends:
| Product | Where licensed | Esters per 1 mL | Source |
|---|---|---|---|
| Sustanon 250 | UK (first authorized 1973) | Propionate 30 mg, phenylpropionate 60 mg, isocaproate 60 mg, decanoate 100 mg | UK SmPC |
| Omnadren 250 | Poland | Propionate 30 mg, phenylpropionate 60 mg, isocaproate 60 mg, decanoate 100 mg | Polish medicinal products register |
In Sustanon 250, the 60 mg of isocaproate delivers ≈45 mg of testosterone. That is about a quarter of the 176 mg of testosterone the label states for each ampoule. The Polish register lists Omnadren 250 with the same four esters at the same amounts, under ATC code G03BA03, the same code as Sustanon 250.
In the Polish register, a search for isocaproate returned only Omnadren 250 listings. The FDA’s approved-drug database returned no product containing testosterone isocaproate (openFDA Drugs@FDA).
How it behaves in the body
What the blend data show
Because isocaproate is only given in blends, its own contribution to blood testosterone cannot be separated from the other three esters. The blend data are:
| Measure | Result | Source |
|---|---|---|
| Peak total testosterone after one Sustanon 250 dose | ≈70 nmol/L (≈2,000 ng/dL) | UK SmPC |
| Time to peak | ≈24–48 h | UK SmPC |
| Return to lower limit of male normal range | ≈21 days | UK SmPC |
| Total testosterone 24 h after one dose, healthy young men (n = 9) | 19.8 → 81.4 nmol/L | Solheim et al., Front Physiol 2020 |
| LH 24 h after one dose | 3.8 → 1.6 IU/L | Solheim et al. 2020 |
| Estradiol 24 h after one dose | 63.5 → 99.0 pmol/L | Solheim et al. 2020 |
1 nmol/L of testosterone ≈ 28.8 ng/dL. LH (luteinizing hormone) is the pituitary signal that drives the testes’ own testosterone production.
After release, the hormone is ordinary testosterone. The label states that it is converted to dihydrotestosterone (DHT) and estradiol, binds plasma proteins and SHBG (sex hormone-binding globulin) at over 97% in lab tests, and is excreted mainly in urine as conjugates of etiocholanolone and androsterone (UK SmPC).
The intact ester in blood
Anti-doping laboratories can measure the intact isocaproate ester in blood, before esterases cut it off. Finding an intact ester proves that the testosterone came from outside the body.
- Serum and plasma. In a study of 6 volunteers who received a single injection of either Sustanon or testosterone undecanoate, every ester given, including isocaproate, was detected in blood afterwards. The detection time depended on which ester was given (Forsdahl et al., Drug Test Anal 2015). Peak ester concentrations occurred within 24–72 hours, varying between people and with the length of the ester chain, as summarized by a later study (Solheim et al., Front Physiol 2020).
- Dried blood spots. In 9 men given two Sustanon 250 injections, the isocaproate, propionate, phenylpropionate and decanoate esters were all detectable in dried blood spots. Sustanon was detected in every subject for at least 5 days, and three of the esters for up to 14 days (Solheim et al., Drug Test Anal 2022).
These studies were designed to detect use, not to measure a half-life. The published abstracts do not give an isocaproate-specific elimination rate.
Approved medical use
There is no approved use for testosterone isocaproate alone. As a component of Sustanon 250, it is part of a product the UK label approves for (UK SmPC):
- testosterone replacement therapy in men with hypogonadism confirmed by clinical features and blood tests;
- supportive therapy for transgender men (female-to-male).
The labeled adult dose is usually one 1 mL injection every 3 weeks, adjusted to response. Specialist centers have used 1 mL every 2 to 4 weeks in transgender men. The label states that safety and efficacy have not been adequately established in children and adolescents. The full blend is covered on the Sustanon 250 page.
What the research shows
No trial has tested testosterone isocaproate alone in humans, so there are no isocaproate-specific data on muscle, strength, body composition or safety. The studies below all used blends that contain it.
Delayed puberty in boys
An uncontrolled study treated 13 boys with constitutional delayed puberty (a normal-variant late start to puberty) with a combination of three esters: propionate, phenylpropionate and isocaproate. They received 200 mg of testosterone by intramuscular injection 4 times at 3-week intervals, and were followed for 2 years (Büyükgebiz, Turk J Pediatr 1993):
| Measure | Before | After 2 years |
|---|---|---|
| Growth rate | 4.5 cm/year | 8.4 cm/year |
| Predicted adult height | 173.5 cm | 173.3 cm (no meaningful change) |
| Entered puberty | — | All 13 boys |
Without a control group, it is not possible to say how much of this reflects the treatment rather than the boys’ natural, delayed onset of puberty.
A single dose and athletic performance
In a randomized, double-blind, placebo-controlled trial, 19 healthy young men received one Sustanon 250 injection (n = 9) or placebo (n = 10). Even though testosterone was about four times baseline 24 hours later, there was no improvement in jump height, 30-second cycle sprint power, or isometric arm strength (Solheim et al., Front Physiol 2020).
Longer-term studies of the full blend, including bone density, body composition and muscle studies, are summarized on the Sustanon 250 page. For controlled data on testosterone dose and muscle, see testosterone enanthate, which delivers the same hormone.
Side effects and risks
There are no safety data for the isolated ester. Once cleaved, it delivers ordinary testosterone, so the risks are those of testosterone and of the product it is given in.
Labeled adverse reactions of the blend
The Sustanon 250 label lists reactions associated with androgen therapy in general (UK SmPC):
| Frequency | Reactions |
|---|---|
| Common (1 in 100 to 1 in 10) | Weight gain; raised hematocrit (the share of blood made up of red cells), hemoglobin and red cell count |
| Rare | Pulmonary oil microembolism (oil reaching the lungs during or right after injection, causing cough, breathlessness, chest pain or fainting) |
| Frequency not known | Polycythemia, fluid retention, high blood pressure, mood changes, depression, libido changes, nausea, abnormal liver tests, acne, gynecomastia, low sperm count, priapism, prostate growth, raised PSA, lipid changes |
The label notes that side effects cannot be reversed quickly by stopping, because the depot keeps releasing hormone. In women, it warns of virilization, including hoarseness, acne, excess hair growth, menstrual changes and hair loss.
Hormonal suppression
External testosterone shuts down the body’s own production. In the Danish trial, LH fell by more than half within 24 hours of one Sustanon 250 injection (Solheim et al., Front Physiol 2020). The label lists low sperm count among the reactions.
Breast cancer report with long-term Omnadren
A Polish clinic followed 45 men with hypergonadotropic hypogonadism (testicular failure) treated with Omnadren 250 every 3–4 weeks for 5–26 years. Breast cancer was diagnosed in 2 of them, after 11 and 15 years of therapy. The authors raised a possible link with long-term androgen replacement (Medras et al., Breast Cancer Res Treat 2006). This was a small case series with no comparison group, so it cannot establish cause. The Sustanon 250 label lists known or suspected breast cancer as a contraindication.
Who should not use the blends
The Sustanon 250 label contraindicates it in:
- pregnancy and breastfeeding;
- known or suspected prostate or breast cancer;
- allergy to any ingredient, including the refined peanut (arachis) oil carrier, and in people allergic to peanuts or soya.
Each mL also contains 100 mg of benzyl alcohol, and the label says it must not be given to premature babies or newborns.
Legal status and use outside medicine
- US. No FDA-approved product contains testosterone isocaproate (openFDA Drugs@FDA). Testosterone, including its salts, esters and ethers, is a Schedule III anabolic steroid (21 CFR 1308.13).
- Sport. Testosterone is listed under S1.1 anabolic androgenic steroids, together with “their esters,” and is prohibited at all times, in and out of competition (WADA 2026 Prohibited List). The Sustanon 250 label warns athletes that it can interfere with anti-doping tests.
- Detection. Beyond blood tests, the intact ester has been found in hair. In a forensic case of a 29-year-old man who died after long-term use of several anabolic steroids, segmental hair analysis detected testosterone isocaproate along with many other steroids and esters (Gheddar et al., J Anal Toxicol 2023). A single case involving many drugs cannot link the death to any one compound.
- Quality. Products sold outside pharmacies are not regulated. In an analysis of 40 seized anabolic steroid products, including ones labeled as containing testosterone isocaproate, 8 did not contain the labeled active ingredient, and others were adulterated or underdosed; sildenafil was found in 4 (Ribeiro et al., Steroids 2018).
Key takeaways
- A mid-length ester, not a standalone drug. Testosterone isocaproate carries a six-carbon branched ester and is about 75% testosterone by weight. In the registers we checked, it appears only in four-ester blends.
- 60 mg per mL in Sustanon 250 and Omnadren 250. That supplies ≈45 mg of the 176 mg of testosterone in each 1 mL ampoule of Sustanon 250.
- No verified half-life of its own. No human study has measured the kinetics of the isolated ester. The blend peaks at 24–48 hours and returns to the low-normal range in about 3 weeks.
- Risks are those of testosterone. Raised hematocrit, suppression of the body’s own testosterone and fertility, lipid changes and virilization in women are labeled effects of the blend.
- Controlled and banned. It is Schedule III in the US as a testosterone ester and prohibited at all times under WADA rules. The intact ester can be detected in blood and hair.
Sources
- Aspen Pharma Trading Limited. Sustanon 250, 250mg/ml solution for injection: Summary of Product Characteristics. PL 39699/0059; revised December 2025. emc.
- Office for Registration of Medicinal Products, Medical Devices and Biocidal Products (Poland). Register of Medicinal Products: Omnadren 250, solution for injection (30 mg + 60 mg + 60 mg + 100 mg)/ml, registration no. 03193. Polish register.
- National Center for Biotechnology Information. PubChem compound summaries: testosterone isocaproate (CID 231084), testosterone (CID 6013), testosterone propionate (CID 5995), testosterone phenylpropionate (CID 14743), testosterone enanthate (CID 9416), testosterone cypionate (CID 441404), testosterone decanoate (CID 155143). PubChem.
- Kicman AT. Pharmacology of anabolic steroids. Br J Pharmacol. 2008;154(3):502–521. PubMed.
- Forsdahl G, Erceg D, Geisendorfer T, et al. Detection of testosterone esters in blood. Drug Test Anal. 2015;7(11-12):983–989. PubMed.
- Solheim SA, Mørkeberg J, Juul A, et al. An intramuscular injection of mixed testosterone esters does not acutely enhance strength and power in recreationally active young men. Front Physiol. 2020;11:563620. PubMed.
- Solheim SA, Levernaes MCS, Mørkeberg J, et al. Stability and detectability of testosterone esters in dried blood spots after intramuscular injections. Drug Test Anal. 2022;14(11-12):1926–1937. PubMed.
- Büyükgebiz A. Treatment of constitutional delayed puberty with a combination of testosterone esters. Turk J Pediatr. 1993;35(4):271–275. PubMed.
- Medras M, Filus A, Jozkow P, Winowski J, Sicinska-Werner T. Breast cancer and long-term hormonal treatment of male hypogonadism. Breast Cancer Res Treat. 2006;96(3):263–265. PubMed.
- Gheddar L, Blanchot A, Ameline A, Arbouche N, Raul JS, Kintz P. The power of keratinous matrices (head hair, body hair and nail clippings) analysis in a case of death involving anabolic agents. J Anal Toxicol. 2023;46(9):e307–e313. PubMed.
- Ribeiro MVM, Boralle N, Felippe LG, Pezza HR, Pezza L. ¹H NMR determination of adulteration of anabolic steroids in seized drugs. Steroids. 2018;138:47–56. PubMed.
- US Food and Drug Administration. Drugs@FDA dataset, queried via openFDA for the active ingredient testosterone isocaproate (no matches). openFDA.
- Drug Enforcement Administration. Schedules of controlled substances: Schedule III, paragraph (f), anabolic steroids. 21 CFR 1308.13. eCFR.
- World Anti-Doping Agency. World Anti-Doping Code International Standard: Prohibited List 2026. WADA 2026 Prohibited List.