Testosterone

Testosterone is the body’s main androgen (the main male sex hormone), and it’s the molecule the whole family of anabolic-androgenic steroids (AAS) is built from. It’s a steroid, not a peptide. Doctors have prescribed it for decades as replacement therapy for men whose bodies don’t make enough — a condition called hypogonadism — and the proof that it builds muscle and strength is some of the strongest we have for any performance drug. That said, it’s a Schedule III controlled substance, it’s banned in sport at all times, and it carries genuine risks to the heart, blood, hormones, and (with certain pills) the liver. This page is here to inform and reduce harm. It contains no doses, cycles, ester schedules, stacks, post-cycle protocols, or sourcing information.

What it is

Testosterone is the body’s main natural androgen — a C19 steroid (4-androsten-17β-ol-3-one), and the original anabolic-androgenic steroid that all the others are synthetic copies of. Here’s how it works: it switches on the androgen receptor (AR) (a protein inside cells that responds to male hormones). Once testosterone latches onto that receptor, the pair moves into the cell’s nucleus and changes which genes are turned on. That’s what drives muscle growth (the anabolic effects) and male sexual characteristics (the androgenic effects). Testosterone is also a starting material the body converts into other hormones: an enzyme called 5α-reductase turns it into dihydrotestosterone (DHT, a stronger androgen that acts on the prostate, skin, and hair), and another enzyme, aromatase (CYP19), turns it into estradiol (a form of estrogen that helps bone and libido but can also cause side effects like breast tissue growth).

The medical forms work in different ways:

• Plain testosterone (with nothing attached) clears out of the body too fast to work as a pill, so it’s given through the skin (gels and patches), as a product absorbed in the cheek or nose, or as a pellet placed under the skin.
• Injectable esters (such as enanthate, cypionate, and undecanoate) are testosterone with a fatty chain attached at one spot on the molecule (the 17β-hydroxyl). The body slowly snips off that chain to release plain testosterone, which stretches out the dose from an oil depot so it lasts longer. This attachment is just a delivery trick — it does not make the drug harmful to the liver.
• 17α-alkylated (17-aa) pills (such as methyltestosterone) have a small chemical group (a methyl group at the C17 position) added so the liver can’t break them down on the first pass. That’s what lets them work as a pill — but it comes at the cost of liver damage that gets worse with higher doses. The injectable testosterone esters are not 17α-alkylated and don’t carry this liver risk. Most testosterone therapy today is not the 17-aa kind.

The claims

Medical: Testosterone is approved and used as replacement therapy for men with confirmed hypogonadism, and it’s also used for delayed puberty in boys and a few other hormone-related conditions. (Some use in gender-affirming care and select conditions in women falls outside what this page covers.)

Athletic and bodybuilding (non-medical): Testosterone is used off-label, usually without a prescription, to add lean mass and strength and to recover faster. This kind of use is banned in sport.

What the evidence actually shows

The evidence that testosterone builds muscle and strength in people is unusually solid.

• High-dose testosterone builds muscle and strength even without exercise, and adds even more on top of training. In a landmark randomized controlled trial (a study where people are randomly assigned to the drug or a dummy treatment) — Bhasin et al., NEJM 1996; 43 healthy men, 10 weeks — the testosterone groups gained fat-free mass, bigger muscles, and more bench-press and squat strength than the placebo group; testosterone plus exercise gave the biggest gains of all (PMID 8637535).
• The more you take, the bigger the effect. Studies testing several dose levels found that fat-free mass, muscle size, and leg-press strength and power all rose along with the dose (Bhasin et al., Am J Physiol Endocrinol Metab 2001; PMID 11701431).
• Older men respond too. In healthy older men, testosterone increased lean mass and muscle strength and power, and older men gained just as much as younger men across the same range of doses (Bhasin et al., J Clin Endocrinol Metab 2005; PMID 15562020).
• In men with low testosterone and in older men, replacement gives modest but real gains in lean mass, muscle strength, and some measures of physical and sexual function — but the payoff for harder-to-measure things (like energy and thinking) is smaller and less consistent. The main evidence here is the Testosterone Trials (TTrials), a coordinated set of studies in men aged 65 and older with low testosterone (Snyder et al., NEJM 2016; PMID 26886521).

Bottom line on whether it works: strong, repeatable evidence for muscle and strength gains that grow with the dose; the benefits for hard clinical and functional outcomes are more modest and depend on the specific situation.

Legal and regulatory status

FDA (US): Testosterone is an FDA-approved prescription drug. It’s approved for one purpose: testosterone-replacement therapy for male hypogonadism — confirmed low testosterone caused by problems with the testes, pituitary, or hypothalamus (the classic, physical kind of hypogonadism). It is not approved for “age-related” low testosterone (the unexplained kind that comes with getting older). In a March 2015 Drug Safety Communication, the FDA spelled out that testosterone products are approved only for men whose low testosterone comes from certain medical conditions, that the benefits and safety haven’t been proven for low testosterone caused by aging alone, and it required every product’s label to say so.

2025 update: After the TRAVERSE trial and FDA-required studies that tracked blood pressure over a full day (ambulatory blood pressure monitoring, or ABPM), the FDA made class-wide label changes on February 28, 2025. It removed the heart-risk language from the boxed warning (the most serious type of label warning), added the TRAVERSE results, kept the note that the drug isn’t for age-related low testosterone, and added a standardized warning about increased blood pressure (which the ABPM studies confirmed across the whole drug class).

DEA scheduling: Testosterone and the other anabolic-androgenic steroids are Schedule III controlled substances under the Controlled Substances Act, set by the Anabolic Steroid Control Act of 1990 (broadened by the Anabolic Steroid Control Act of 2004). Having or distributing them without a prescription is a federal crime. For comparison with drugs people sometimes group together with it: clenbuterol isn’t FDA-approved for humans and is not a federally scheduled controlled substance, and human growth hormone (HGH) has its own law, 21 U.S.C. § 333(e), which makes it a crime to distribute it — or to have it intending to distribute it — for uses it isn’t approved for.

Anti-doping (WADA): Testosterone from outside the body is a Class S1 Anabolic Agent — banned at all times (both in and out of competition). The 2026 Prohibited List spells out that esters of banned anabolic agents (such as testosterone cypionate and propionate) are banned too. To catch it, anti-doping uses the steroid section of the Athlete Biological Passport along with carbon-isotope-ratio (IRMS) testing, which can tell apart testosterone that came from outside the body versus the body’s own. (For context, clenbuterol is S1.2; HGH and hCG are S2 Peptide Hormones/Growth Factors.)

Safety

Strong evidence that something works doesn’t make it harmless — and high-dose, non-medical use has been studied far less than medical replacement therapy, so it’s assumed to be riskier.

• Heart and blood pressure: The large TRAVERSE trial (5,246 middle-aged and older men with low testosterone who were already at high heart risk) found testosterone gel was no worse than placebo for major heart events (the main measure: 7.0% vs 7.3%; HR 0.96) — but it came with higher rates of atrial fibrillation (an irregular heartbeat), acute kidney injury, and pulmonary embolism (a blood clot in the lungs) (Lincoff et al., NEJM 2023; PMID 37326322). Earlier, the TOM trial in older men with limited mobility was stopped early because the testosterone group had a lot more heart-related problems (23 vs 5) (Basaria et al., NEJM 2010; PMID 20592293). The FDA-required ABPM studies confirmed a class-wide rise in blood pressure. The upshot: at replacement doses, the risk of major heart events doesn’t look raised, but the blood-pressure increase and the signals for irregular heartbeat and clots are real.
• Cholesterol: Androgens, especially the oral 17α-alkylated forms, lower HDL (“good”) cholesterol and can raise LDL (“bad”) cholesterol; the injectable and through-the-skin forms affect cholesterol less than the 17-aa pills do.
• Liver damage: This is a worry specifically with the 17α-alkylated oral androgens like methyltestosterone — it can cause cholestatic jaundice (yellowing of the skin from blocked bile flow), raised liver enzymes, and rarely peliosis hepatis (blood-filled cysts in the liver) and liver tumors. Injectable testosterone esters and through-the-skin testosterone do not carry this 17-aa liver risk.
• Shutting down your own hormones and fertility: Taking testosterone from outside tells the body’s own hormone control loop (the hypothalamic-pituitary-gonadal axis) to ease off. That lowers two signaling hormones, LH and FSH, which in turn shuts down sperm production (often all the way to none, called azoospermia) and shrinks the testes. Fertility can take months to come back and doesn’t always fully recover.
• Thicker blood (erythrocytosis): Testosterone tells the body to make more red blood cells; a rise in hematocrit and hemoglobin (polycythemia) — basically thicker blood — is one of the most common side effects and a known clotting risk, which is why checking hematocrit regularly is standard.
• Gynecomastia (breast tissue growth in men): This comes from testosterone being converted into estradiol.
• Virilization (in women) and other androgenic effects: A deeper voice, extra body hair (hirsutism), enlargement of the clitoris, and disrupted periods — these are often permanent; acne and male-pattern hair loss can happen in both men and women.
• Prostate: Testosterone stimulates prostate tissue, can raise PSA (a prostate blood marker) and worsen urinary symptoms, and should not be used by men with known prostate or breast cancer. Long-term data on prostate cancer risk are still limited — reassuring so far, but not the final word.
• Other: Mood and behavior changes and aggression at high doses, fluid retention, worsening sleep apnea, reactions at injection sites, and dependence and withdrawal with long-term high-dose misuse.

Bottom line

Testosterone is an anabolic-androgenic steroid, not a peptide. The human evidence that it adds lean mass and strength — more with higher doses, in both young and older men, with or without training — is strong and repeatable, while its benefits for hard clinical and functional outcomes are more modest and depend on the situation. It’s FDA-approved only for confirmed hypogonadism (not for age-related low testosterone), it’s a Schedule III controlled substance, and it’s banned in sport at all times. Its risks are real: higher blood pressure, signals for irregular heartbeat and blood clots, thicker blood, suppression of the body’s own testosterone and fertility, breast tissue growth in men, prostate effects, masculinizing changes in women, and — with the 17α-alkylated oral forms specifically — liver damage.

Evidence grade: 10/10 · Established.

Sources

• Bhasin S et al. The effects of supraphysiologic doses of testosterone on muscle size and strength in normal men. NEJM 1996 (PMID 8637535)
• Bhasin S et al. Testosterone dose-response relationships in healthy young men. Am J Physiol Endocrinol Metab 2001 (PMID 11701431)
• Bhasin S et al. Older men are as responsive as young men to the anabolic effects of graded doses of testosterone. J Clin Endocrinol Metab 2005 (PMID 15562020)
• Snyder PJ et al. Effects of Testosterone Treatment in Older Men (The Testosterone Trials). NEJM 2016 (PMID 26886521)
• Basaria S et al. Adverse Events Associated with Testosterone Administration (TOM trial). NEJM 2010 (PMID 20592293)
• Lincoff AM et al. Cardiovascular Safety of Testosterone-Replacement Therapy (TRAVERSE). NEJM 2023 (PMID 37326322)
• FDA Drug Safety Communication: FDA cautions about using testosterone products for low testosterone due to aging (March 2015)
• FDA. FDA issues class-wide labeling changes for testosterone products (Feb 28, 2025)
• Anderer S. FDA Updates Testosterone Labeling for Blood Pressure and Cardiovascular Risks. JAMA 2025 (PMID 40184062)
• DEA. Anabolic Steroids (Schedule III; Anabolic Steroid Control Act of 1990/2004)
• WADA. 2026 Prohibited List
• USADA. 2026 WADA Prohibited List summary