Anabolic steroidAndrogenTestosterone esterInjectable

Testosterone Isocaproate

Also known as Test Iso · Isocaproate

Written by enwyReviewed 15 Jul 20262 sources0 comments

A medium-acting injectable testosterone ester, encountered chiefly as one of the four esters in Sustanon. Once cleaved it is bioidentical testosterone; the isocaproate ester gives an intermediate release rate between phenylpropionate and decanoate.

01 Overview

Testosterone isocaproate is a six-carbon branched-chain ester with release kinetics intermediate among the testosterone esters — slower than propionate and phenylpropionate but faster than decanoate. It is almost never sold as a standalone product; its main relevance is as a 60 mg/mL component of Sustanon 250.

Like all testosterone esters, the isocaproate governs only the release rate; the freed hormone is ordinary testosterone, so the effect and side-effect profile is testosterone's. Its distinguishing feature is simply where it sits on the release-duration spectrum.

02 Mechanism

Esterases cleave the isocaproate ester to release free testosterone at an intermediate rate; the hormone activates the androgen receptor, aromatises to estradiol, and suppresses gonadotropins by negative feedback.

03 Dosing

Tier	Dose	Route	Notes
Common	300–500 mg/wk	IM	Usually delivered as part of a blend; injected once or twice weekly.
Strong	500–700 mg/wk	IM	Higher doses scale side effects proportionally.

Ester comparison

Ester	Half-life	Injection freq.	Testosterone by mass

04 Effects

Effect	Magnitude	Evidence
Increased lean massDose-dependent gain in fat-free mass, identical to any testosterone form.	+3-6 kg over a cycle	Clinical
Increased strengthSupraphysiological testosterone raises maximal strength.		Clinical
Improved libido and moodRaising serum testosterone improves libido and mood.		Clinical

05 Side effects

Effect	Severity	Frequency	Evidence
Estrogenic effectsGynecomastia and water retention from aromatisation to estradiol.	Moderate	Common at higher doses	Clinical
ErythrocytosisRaised hematocrit from androgen-stimulated erythropoiesis.	Moderate	Common	Clinical
HPTA suppressionSuppression of LH/FSH, endogenous production and spermatogenesis.	Severe	Universal	Clinical

06 Legal status

United Kingdom

Class C

as of Jan 2026

United States

Schedule III controlled substance

Prescription required; used within blends such as Sustanon. · as of Jan 2026

07 References

Sustanon 250 Summary of Product Characteristicselectronic Medicines Compendium (UK)

Testosterone dose-response relationships in healthy young menAm J Physiol Endocrinol Metab, 2001

08 Discussion0 comments

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This page is reference information, not medical advice. Doses and protocols are documented as they appear in the clinical literature and in practice — describing them is not a recommendation to use them. Countermeasures listed here are not a substitute for a physician. Legal status varies by jurisdiction and changes.