Testosterone
Testosterone is the main male sex hormone. Injectable testosterone esters are prescription medicines approved to treat male hypogonadism, where low testosterone has been confirmed by symptoms and blood tests.
Reference information, not medical advice. Enhanced Training Studio doesn't sell, supply or recommend any substance. Doses are left out on purpose: only a prescriber can set one. How this library is written.
What it is
Testosterone is the main male sex hormone. As a medicine it is usually given as an ester, a form of testosterone joined to a fatty acid chain so that it is released slowly from an oil-based injection. The injectable esters covered on this page are testosterone cypionate, testosterone enanthate (spelled enantate in the UK), testosterone propionate and testosterone undecanoate. Some products, such as Sustanon 250, contain a mix of esters including testosterone propionate. Testosterone is also available as gels, patches and other forms, which are not covered here.
How it works
Testosterone and other androgens bind to androgen receptors inside cells. The receptor then moves to the nucleus and switches on genes involved in male sexual development, muscle, bone and red blood cell production [11].
The ester mainly changes how long the medicine lasts. For a mixed-ester product, UK product information reports peak testosterone levels about 24 to 48 hours after an injection [4]. Testosterone enanthate reaches peak levels at about 24 hours [3]. Testosterone undecanoate injection is a much longer-acting depot [5].
Approved medical uses
- Male hypogonadism. In the UK, testosterone enantate, Sustanon 250 and Nebido (testosterone undecanoate) are licensed for testosterone replacement in men whose deficiency has been confirmed by clinical features and blood tests [3, 4, 5].
- US labelling. Depo-Testosterone (testosterone cypionate) is approved for primary hypogonadism and hypogonadotropic hypogonadism. The label states that safety and efficacy in "age-related hypogonadism" have not been established [2]. Aveed (testosterone undecanoate injection) was approved by the FDA in 2014 with a restricted-access safety programme [6].
- Other licensed uses. The UK product information for Sustanon 250 also lists supportive therapy for female-to-male transgender people [4].
Guidelines recommend diagnosing hypogonadism only when there are consistent symptoms and consistently low testosterone levels on repeated blood tests [1].
What the research shows
- Cardiovascular safety. The TRAVERSE trial randomised 5,246 men aged 45 to 80 with low testosterone and existing or high risk of cardiovascular disease to testosterone or placebo. Over an average of about 33 months, testosterone was non-inferior to placebo for major heart events (7.0% vs 7.3%) [8]. In 2025 the FDA removed the boxed warning about cardiovascular risk from testosterone labels and added the TRAVERSE results [7].
- Other findings from TRAVERSE. Atrial fibrillation, acute kidney injury and pulmonary embolism were more common with testosterone than placebo [8].
- Blood pressure. FDA-required ambulatory blood pressure studies found that testosterone raises blood pressure across all products, and labels now carry this warning [7].
- Older men with low testosterone. The Testosterone Trials, a group of placebo-controlled trials in 790 men aged 65 and over treated for one year, found improvements in sexual function, mood, bone density and anaemia, but no improvement in memory or cognition. Testosterone was linked to a greater increase in coronary artery plaque volume on CT scans. The investigators said the trials were too small and short to judge cardiovascular risk [9].
Known risks and side effects
- Raised red blood cell count (polycythaemia). This is a common effect, and a raised haematocrit is a reason not to start treatment [1, 2, 5].
- Raised blood pressure [2, 7].
- Blood clots in the veins and lungs have been reported [2, 8].
- Prostate effects. Testosterone can enlarge the prostate, and older men may be at higher risk of prostate enlargement and prostate cancer [2]. Testosterone should not be used in men with known or suspected prostate or breast cancer [1, 4].
- Reduced sperm production and fertility. Testosterone can suppress sperm production [2, 3], and guidelines advise against it in men planning a pregnancy with their partner soon [1].
- Pulmonary oil microembolism (POME) and allergic reactions. Oil-based injections, especially testosterone undecanoate, can rarely cause cough, breathlessness and chest pain during or just after the injection, and severe allergic reactions have been reported [3, 5, 6].
- Liver effects. Prolonged use of androgens at high doses has been linked to liver tumours and peliosis hepatis (blood-filled cysts in the liver) [2].
- Other effects include fluid retention, which can be serious in people with heart, kidney or liver disease, and breast enlargement [2]. Guidelines advise against starting testosterone in men with untreated severe sleep apnoea [1].
- Abuse and dependence. Misuse at high doses is linked to serious heart and psychiatric effects [2].
- Testosterone is prohibited at all times in sport under the World Anti-Doping Agency (WADA) Prohibited List [10].
Testosterone should only be used under medical supervision.
Blood tests commonly monitored
- Testosterone level, to check it is in the normal range on treatment [1, 5].
- Haematocrit and haemoglobin, before starting and periodically on treatment [1, 2, 3, 5].
- PSA (prostate-specific antigen), with a prostate examination, in men for whom prostate monitoring is appropriate [1, 4, 5].
- Lipid profile (cholesterol), as cholesterol may change on treatment [2, 4, 5].
- Liver function tests [2, 4, 5].
- Blood pressure, which is not a blood test but is advised on labels [2, 7].
If you and your doctor are monitoring treatment, Enhanced Training Studio keeps a private log on your phone: what you take and when, your blood test results over time, and how you feel. How tracking works.
Sources
- Bhasin S, Brito JP, Cunningham GR, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. Journal of Clinical Endocrinology and Metabolism, 2018. Endocrine Society
- Pfizer. DEPO-Testosterone (testosterone cypionate) injection: US prescribing information, revised 2025. Pfizer labeling
- Electronic Medicines Compendium. Testosterone Enantate Solution for Injection Ampoules: Summary of Product Characteristics, 2025. emc
- Electronic Medicines Compendium. Sustanon 250: Summary of Product Characteristics, 2025. emc
- Electronic Medicines Compendium. Nebido (testosterone undecanoate) solution for injection: Summary of Product Characteristics, 2025. emc
- US Food and Drug Administration. Aveed (testosterone undecanoate) injection, NDA 22-219: Summary Review, 2014. FDA
- US Food and Drug Administration. FDA issues class-wide labeling changes for testosterone products, 2025. FDA
- Lincoff AM, et al. Cardiovascular Safety of Testosterone-Replacement Therapy (TRAVERSE). New England Journal of Medicine, 2023. doi.org/10.1056/NEJMoa2215025
- Penn Medicine. Testosterone treatment improves bone density and anemia, may lead to cardiac risk (Testosterone Trials results), 2017. Penn Medicine
- World Anti-Doping Agency. The 2026 Prohibited List, International Standard, as published in the Treaty Series of the Kingdom of the Netherlands (Tractatenblad 2026, No. 25). Officielebekendmakingen.nl
- National Library of Medicine. Androgenic Steroids. LiverTox: Clinical and Research Information on Drug-Induced Liver Injury, NCBI Bookshelf, updated 2020. NCBI Bookshelf
Related
Last reviewed 7 October 2026. General information, not medical advice. Found a mistake? Email [email protected].