Educational reference only. Informational material drawn from peer-reviewed literature. Not medical advice and not a recommendation to use, dose, or modify any therapy. Decisions about hormone therapy should be made with a qualified healthcare provider.
Overview
Overview
Testosterone is the principal androgen hormone produced naturally by the testes in men and, in smaller amounts, by the ovaries and adrenal glands in women. Synthetic testosterone is used clinically as testosterone replacement therapy (TRT) in men with hypogonadism, a condition where natural production is insufficient. It is FDA-approved, has a well-characterized safety profile, and is the most-studied anabolic-androgenic compound in clinical medicine.
The molecule itself has the same biological activity whether produced endogenously or administered exogenously. What varies is the ester attached to it, a fatty-acid chain that controls how slowly the body releases active testosterone after injection.
Once cleaved from the ester, the testosterone molecule itself has a much shorter half-life of approximately 10–100 minutes in circulation. The ester is purely a release-rate modifier: it does not change what testosterone does in the body, only how quickly the dose becomes available.
Forms
Available forms & half-lives
| Form | Approximate half-life |
|---|---|
| Suspension (no ester) | <1 day |
| Propionate | ~2 days |
| Phenylpropionate | ~1.5 days |
| Enanthate | ~4.5 days |
| Cypionate | ~5 days (functionally equivalent to enanthate) |
| Undecanoate (in oil) | ~21 days |
What it does
Main effects
- Increased lean body mass and muscle strength, dose-dependent PMID: 11701431
- Improved libido, erectile function, and sexual response (when treating hypogonadism)
- Mood improvement and reduced fatigue in hypogonadal men
- Increased bone mineral density in hypogonadal men returning to physiologic range
- Improved insulin sensitivity; 22.5% relative reduction in new-onset diabetes in TRAVERSE PMID: 37326322
- Stimulates erythropoiesis (basis for the original anemia-of-chronic-disease indication)
- Maintains male secondary sexual characteristics (body hair, voice, libido) in replacement therapy
What to watch for
Common side effects
- Erythrocytosis: elevated hematocrit, the most common adverse effect (5–66% across studies, higher with injectables) PMID: 27784544
- HPG axis suppression: testicular atrophy, suppressed LH/FSH, contraceptive in most men on therapeutic doses
- Estradiol elevation: water retention, gynecomastia risk; worse at higher body fat
- Acne and oily skin: dose-dependent
- Scalp hair loss: in genetically predisposed individuals (mediated through DHT)
- Sleep apnea worsening: particularly at supraphysiologic doses
- Fracture risk: small but statistically significant increase observed in TRAVERSE substudy PMID: 37326322
Key facts
Key facts worth knowing
- The most rigorous cardiovascular safety study to date (the TRAVERSE trial of 5,246 hypogonadal men with elevated cardiovascular risk) found that testosterone therapy was non-inferior to placebo for major adverse cardiac events over a median 3.2-year follow-up. PMID: 37326322
- The most common adverse effect is erythrocytosis (elevated red blood cell count), which occurs in 5–66% of men on testosterone therapy depending on formulation and follow-up duration. PMID: 27784544
- Injectable forms cause greater erythrocytosis than transdermal gels. PMID: 27784544
- The TRAVERSE substudy also found a small but statistically significant increase in fracture risk with testosterone therapy versus placebo, a counterintuitive finding given testosterone's anabolic bone effects, and one still being investigated. Cleveland Clinic, 2024
Legal status
Legal status
Schedule III controlled substance in the United States (DEA). Prescription-only. WADA prohibits exogenous testosterone in competitive sport at all times.