Testosterone cypionate

What it is

Testosterone cypionate is an intramuscularly injected ester of the natural androgen testosterone, prescribed as replacement therapy for men with clinically and biochemically confirmed hypogonadism. It's one of three FDA-approved injectable testosterone esters in the US — alongside testosterone enanthate and testosterone undecanoate — and remains among the most widely prescribed forms of TRT because of its long track record, low cost, and convenient dosing interval.¹

Also known as: Depo-Testosterone (the original branded product), or simply "test cyp" in clinical shorthand. It's sold under multiple generic labels ("Testosterone Cypionate Injection, USP") and can also be prepared by 503A compounding pharmacies.¹

Testosterone cypionate isn't a synthetic androgen in the sense of an anabolic steroid designed to diverge from natural testosterone — it is testosterone, chemically modified only enough to slow its release from an injection site so it doesn't need to be dosed multiple times a day. Once that modification is undone by the body, what circulates is identical to the testosterone a healthy testis produces.

How it works

Testosterone cypionate is a prodrug. The parent androgen is esterified at its 17β-hydroxyl position with cyclopentylpropionic acid, which sharply increases its lipophilicity and lets it dissolve in an oil vehicle (cottonseed oil, USP). After a deep intramuscular or subcutaneous injection, the ester slowly partitions out of that oil depot into the bloodstream, where non-specific esterases in plasma and tissue cleave the cyclopentylpropionate side chain and release free, bioactive testosterone.¹ Native, unesterified testosterone has a plasma half-life of only about 10 minutes — esterification is what makes an injectable product practical at all.

Once liberated, testosterone acts through several parallel pathways: it binds the androgen receptor directly, driving transcriptional changes in muscle, bone, and brain tissue; it's converted by 5α-reductase into dihydrotestosterone (DHT), a more potent androgen that dominates effects on the prostate and hair follicles; and it's aromatized to estradiol in fat tissue, which matters for bone density, libido, and feedback signaling back to the brain. That last pathway is also the reason exogenous testosterone suppresses the body's own hormone axis — rising testosterone and estradiol both signal the hypothalamus and pituitary to dial back LH and FSH, which is why TRT alone tends to shut down natural testicular production and sperm output.

Pharmacokinetically, a single injection produces a "saw-tooth" pattern rather than a steady physiologic rhythm: serum testosterone climbs to a supraphysiologic peak within 3–5 days, then falls gradually until the next dose. The ester's apparent half-life is roughly 8 days — a touch longer than enanthate's ~7 days because its side chain is slightly more lipophilic — so steady state takes about 5–6 weeks of consistent dosing to reach.¹¹⁷ While gluteal intramuscular injection is the classic route, subcutaneous injection into abdominal fat is increasingly used off-label because it produces comparable testosterone levels with less injection-site discomfort.

What the research shows

Animal / preclinical evidence

Testosterone's androgen-receptor pharmacology has been characterized in animal and cell-culture models for decades, but current clinical guidance for cypionate rests almost entirely on human trial data rather than preclinical work — androgen physiology is one of the best-studied endocrine systems, and the open questions that matter today (cardiovascular risk, fracture risk, prostate safety) can only be meaningfully answered in humans.

Human evidence

The largest and most consequential trial is TRAVERSE, a randomized, double-blind, placebo-controlled, FDA-mandated postmarketing trial in 5,246 men aged 45–80 with confirmed hypogonadism and elevated cardiovascular risk.⁶ Over a mean of nearly two years, testosterone (as transdermal gel) was noninferior to placebo for the composite of cardiovascular death, heart attack, or stroke (7.0% vs 7.3%; HR 0.96). That finding led the FDA to remove testosterone's cardiovascular boxed warning in February 2025.¹⁵ But TRAVERSE wasn't an unambiguous clean bill of health: it also found significantly more atrial fibrillation (3.5% vs 2.4%) and acute kidney injury (2.3% vs 1.5%) in the testosterone group, plus a non-significant trend toward more blood clots.

A prespecified TRAVERSE bone substudy delivered a genuinely counterintuitive result: despite testosterone increasing bone mineral density — a positive finding from earlier trials — the group taking testosterone actually had more clinical fractures than placebo (3.5% vs 2.46%; HR 1.43).⁸ A separate TRAVERSE prostate-safety substudy of over 5,200 men found no significant difference in high-grade prostate cancer, though PSA rose modestly more on testosterone — an expected effect of the drug working, not necessarily a cancer signal.⁷

Older trials tell a more mixed efficacy story. The Testosterone Trials (790 men over 65) found modest improvements in sexual activity, desire, and erectile function, and better mood, but no significant benefit for walking distance or vitality in most participants.⁹ A 2018 meta-analysis of four low-risk-of-bias RCTs (N=1,779) found small but real improvements in libido and sexual satisfaction, no effect on energy or mood, and an eightfold increase in erythrocytosis (abnormally high red blood cell counts) versus placebo.¹¹ A smaller, earlier trial in frail older men was stopped early after a significantly higher rate of cardiovascular events in the testosterone group — a cautionary finding that was not replicated by the much larger TRAVERSE trial but is still worth knowing about.¹²

Honest bottom line: the efficacy signal for testosterone therapy is real but modest, and concentrated mostly in sexual function — not the broad energy, mood, and vitality improvements sometimes assumed. TRAVERSE substantially de-risked the cardiovascular death/MI/stroke question, but it also revealed new signals (atrial fibrillation, kidney injury, a possible clotting risk, and a fracture-risk paradox) that deserve ongoing attention rather than dismissal.

Typical use and dosing

FDA labeling allows 50–400 mg intramuscularly every 2–4 weeks, individualized to keep testosterone within the normal physiologic range.³ In the TRAVERSE trial, dosing (as transdermal gel) was titrated to a trough target of 350–750 ng/dL. In everyday clinical practice, many prescribers favor smaller, more frequent doses — for example, weekly or twice-weekly injections rather than every 2–4 weeks — to smooth out the peak-to-trough swings that come with less frequent dosing and potentially reduce erythrocytosis and mood fluctuation. That approach has pharmacokinetic logic behind it but hasn't been tested head-to-head against standard dosing in large trials.

Once a diagnosis of persistent hypogonadism is confirmed, TRT is typically continued long-term, but the American Urological Association recommends revisiting the decision to continue treatment 3–6 months in if testosterone has normalized on labs but symptoms haven't improved.¹⁴

Safety, side effects, and who should avoid it

Common side effects reported in trials and labeling include injection-site pain or irritation, acne and oily skin, and increased hematocrit — clinically significant erythrocytosis affects roughly 5–7% of men at standard doses, though a much larger share show some hematocrit rise depending on the threshold used.³ Less common effects (roughly 1–10% of patients) include gynecomastia, mood changes, headache, modestly elevated PSA, sleep apnea worsening, fluid retention, and suppressed sperm production with prolonged use — usually reversible after stopping, though recovery can take months.

Rare but serious events include pulmonary oil microembolism (a vehicle-related reaction that can mimic anaphylaxis right after injection), hepatic effects mostly associated with oral androgens rather than cypionate specifically, hypercalcemia in immobilized patients, and priapism. Drug interactions worth flagging: androgens can potentiate warfarin's anticoagulant effect (requiring closer INR monitoring) and may improve insulin sensitivity, sometimes requiring adjustment of diabetes medications.

Standard monitoring includes a comprehensive baseline panel — morning fasting total and free testosterone, LH, SHBG, hematocrit, PSA (for men over 40), lipids, and metabolic markers; repeat testosterone and hematocrit 3–6 months after starting; and ongoing testosterone checks every 6–12 months, hematocrit annually, PSA per age-appropriate screening, and blood pressure checks starting around 6 weeks in, per the 2025 FDA labeling update.¹³¹⁵

Regulatory status

Testosterone cypionate has been FDA-approved since 1979 (Depo-Testosterone), with generic versions approved through 2022, specifically for replacement therapy in men with confirmed primary or hypogonadotropic hypogonadism.²³ It is explicitly not approved for age-related ("late-onset") hypogonadism — labeling limits use to structural or genetic causes. It's a DEA Schedule III controlled substance under the Anabolic Steroids Control Act, requiring standard controlled-substance prescribing rules.¹⁶

In February 2025, following the TRAVERSE trial and required blood-pressure monitoring studies, the FDA directed a class-wide labeling update: removing the cardiovascular boxed warning while adding a new warning about testosterone-associated blood pressure increases.¹⁵ As a small-molecule steroid rather than a biologic, testosterone cypionate remains eligible for 503A/503B compounding, unlike hCG, which lost that eligibility in 2020.

Frequently asked questions