Testosterone Replacement Therapy: An Evidence-Based Review of What Works, What’s Safe, and What’s Being Oversold

Testosterone therapy is the fastest-growing corner of men’s medicine — and one of the most aggressively marketed. Here is an honest map of how low testosterone is actually diagnosed, every major way it can be treated, what the newest trials say about safety, and where the wellness industry gets ahead of the evidence.

Few therapies illustrate the gap between marketing and medicine better than testosterone. On one side, decades of rigorous research show that for men with true, confirmed testosterone deficiency — hypogonadism — replacing the hormone can meaningfully improve sexual function, body composition, bone strength, mood, and anemia. On the other side, a booming online industry now sells testosterone to men whose real problem is poor sleep, excess weight, or ordinary aging, often after a single low blood draw and a checkout page. Both things are true at once, and that is exactly why this therapy needs a physician rather than an algorithm. This review walks through how deficiency is properly diagnosed, categorizes every major treatment strategy by how strong the evidence is, summarizes what the landmark TRAVERSE cardiovascular-safety trial actually found, and is honest about the real risks — from erythrocytosis to infertility — that get glossed over in the rush to prescribe. Done correctly, TRT is powerful, restorative, and a legitimate tool of preventive medicine. Done carelessly, it is one of the clearest ways the wellness industry can quietly harm the people it claims to help.

Who Actually Has Low Testosterone

Testosterone is made mostly in the testes under signals from the brain (the hypothalamic–pituitary–gonadal axis). Levels naturally decline with age, roughly 1–2% per year after the 30s, but a low number on a lab report is not by itself a disease. The medical societies are unusually aligned on this point: a genuine diagnosis of hypogonadism requires two things together — consistent symptoms (low libido, erectile dysfunction, fatigue, loss of muscle mass, depressed mood, loss of morning erections) and unequivocally low testosterone confirmed on two separate early-morning, fasting blood tests. The American Urological Association sets that threshold at a total testosterone below 300 ng/dL; the Endocrine Society uses a similar cut point and, in a July 2026 statement, reiterated that both symptoms and reliably measured low levels must be present before treatment.

Two details separate careful diagnosis from the rushed version sold online. First, timing and repetition matter: testosterone peaks in the morning and swings day to day, so a single afternoon draw is nearly worthless for diagnosis. Second, the total number can lie. Most testosterone travels bound to a carrier protein, sex hormone–binding globulin (SHBG); when SHBG is high (older men, liver disease) or low (obesity, insulin resistance, hypothyroidism), the total can look normal while the biologically active free testosterone tells a different story. A proper evaluation also checks LH, FSH, and prolactin to locate the problem (testicular vs. pituitary), and screens for the reversible causes — obesity, uncontrolled diabetes, sleep apnea, opioids, alcohol — that so often masquerade as “low T.” In many men, fixing those is the treatment.

What TRT Actually Does — The Evidence for Benefit

The best evidence for TRT’s benefits comes from the coordinated Testosterone Trials (T-Trials) — seven placebo-controlled trials in 790 men over 65 with unequivocally low levels — and from numerous randomized trials since. The honest summary is that benefits are real but selective. Sexual function is the most consistent win: testosterone reliably improved libido, erectile function, and sexual activity versus placebo. Body composition improves dependably too — meta-analyses show increased lean muscle mass and reduced fat mass — and bone density and estimated strength rose, while testosterone corrected anemia, including anemia of unknown cause.

Just as important is what TRT does not reliably do. In the T-Trials, gains in mood and energy were modest, the effect on walking and physical function was small, and there was no meaningful benefit for memory or cognition. Testosterone tends to improve some metabolic markers — it modestly lowers fat mass and can nudge insulin sensitivity — but it is not a weight-loss drug, not a treatment for ordinary age-related tiredness, and not a longevity pill. The men who benefit are those who were genuinely deficient to begin with; giving testosterone to men with normal levels chasing a performance edge is a different activity entirely, with the risks but not the medical justification.

How Testosterone Is Delivered

Injectable testosterone (cypionate & enanthate)

Intramuscular or subcutaneous injections every 1–2 weeks are the oldest, best-studied, and least expensive backbone of therapy. Dosed and monitored well, they are highly effective. Their main trade-off is the peak-and-trough pattern — levels can run high after an injection and dip before the next — which can drive mood swings and, at higher doses, raise the red-blood-cell count (erythrocytosis). Smaller, more frequent subcutaneous dosing has become popular precisely because it smooths these swings. This is a formulation where “more is better” thinking causes real harm, and where physician-set dosing matters.

Transdermal gels and solutions

Daily gels and solutions (e.g., AndroGel, Testim) produce steady, physiologic levels and were the formulation used in the landmark TRAVERSE and T-Trials research, which is part of why their safety data are the most robust. The practical catch is skin-to-skin transfer: testosterone can rub off onto a partner or child, so application sites must be covered and washed. Patches and short-acting buccal and nasal preparations round out the daily options; the nasal form’s short action may disturb the fertility axis less, but adherence to multiple daily doses is a real-world limitation.

Long-acting injections, oral, and pellets

For men who prefer infrequent dosing, long-acting IM testosterone undecanoate is given roughly every 10 weeks but must be administered in a clinic with a period of observation for a rare pulmonary reaction. Modern oral testosterone undecanoate (e.g., Jatenzo) is a genuine advance over the old, liver-toxic oral testosterone of decades past, though its long-term data are thinner and it can raise blood pressure. Subcutaneous pellets, implanted every 3–6 months, deliver very steady levels and excellent convenience — but they are not easily reversible if a problem arises, tend to aromatize more to estrogen, and carry a small risk of extrusion or infection at the implant site. None of these is universally “best”; the right choice depends on the man’s goals, fertility plans, tolerance for needles, and how his body handles each option — which is exactly the kind of judgment a prescription-by-questionnaire cannot make.

Safety, Monitoring & the TRAVERSE Trial

For years, the biggest open question about TRT was the heart. That question now has its best answer. TRAVERSE, published in the New England Journal of Medicine in 2023, randomized 5,246 middle-aged and older men with hypogonadism and either established cardiovascular disease or high risk for it to testosterone gel or placebo, and followed them for about two years. The result: testosterone was noninferior to placebo for major adverse cardiac events — meaning it did not increase heart attacks, strokes, or cardiovascular death. On the strength of this trial, in 2025 the FDA removed the long-standing class-wide boxed warning about cardiovascular risk from testosterone products — while adding a new warning that testosterone can raise blood pressure.

That reassurance is real, but it is not a green light, and honest reporting means stating the fine print. TRAVERSE also found small but statistically significant increases in the testosterone group in atrial fibrillation, pulmonary embolism (blood clots in the lung), and acute kidney injury. And in a linked fracture sub-study, men on testosterone paradoxically had more clinical fractures than placebo (3.5% vs. 2.5%) — a reminder that better bone density on a scan does not always translate into fewer broken bones, and that no hormone is consequence-free. The 2026 Endocrine Society statement folds these findings together: reassuring on heart attack and stroke, but with clot and other signals that mean TRT belongs with men who have a real indication, not everyone who wants it.

The risk almost no online clinic mentions: fertility

Exogenous testosterone signals the brain to stop its own production, which shuts down sperm production and shrinks the testes — and for some men this does not fully reverse after stopping. A younger man who may want children should almost never be started on plain testosterone without this conversation. This single issue is one of the clearest reasons TRT belongs with a physician who asks about family plans before writing a prescription — and it leads directly to the alternatives most direct-to-consumer models never offer.

Adjuncts & Fertility-Sparing Alternatives

hCG — keeping the testes online

Human chorionic gonadotropin (hCG) mimics the brain’s LH signal, directly stimulating the testes to keep producing testosterone and sperm. Used alongside TRT, it prevents or reverses the testicular shrinkage and fertility loss that plain testosterone causes; used alone, it can raise testosterone in men with secondary (pituitary/hypothalamic) hypogonadism without shutting the system down. It requires injections and careful estradiol monitoring, but for the right man it is the difference between preserving fertility and losing it.

Clomiphene and enclomiphene — raising your own testosterone

These selective estrogen receptor modulators (SERMs) block estrogen feedback at the brain, prompting the body to make more of its own testosterone — taken as a pill, without the injections or the fertility shutdown of exogenous testosterone. In a randomized phase II trial, enclomiphene raised testosterone while preserving sperm counts, whereas testosterone gel suppressed them, and a 2025 systematic review found clomiphene and enclomiphene achieve testosterone improvements comparable to TRT in appropriately selected men. For younger men with secondary hypogonadism who want to protect fertility, this is often a better first move than testosterone itself — a genuinely useful option that a questionnaire-based service rarely bothers to offer.

Anastrozole — and where the frontier gets oversold

Aromatase inhibitors such as anastrozole lower the conversion of testosterone to estrogen and have a legitimate, narrow role: men with documented high estradiol and related symptoms. Routinely crushing estrogen to near zero, as some aggressive protocols do, is a mistake — men need estrogen for bone, joints, libido, and mood. At the far end sit SARMs (selective androgen receptor modulators) and over-the-counter “testosterone boosters.” SARMs are genuinely interesting in the lab, but none is FDA-approved, they are sold gray-market as “research chemicals,” products are frequently mislabeled or contaminated, and the FDA has explicitly warned about liver and cardiovascular harms. “T-booster” supplements, for their part, have essentially no convincing human evidence. This is the corner of the field where marketing most outruns data — and where a physician’s willingness to say “not yet, and not for you” is worth more than any product.

The Long View

A hormone worth respecting — and an industry racing ahead of the evidence

Testosterone has become one of the fastest-growing categories in all of consumer health, propelled by telehealth, monthly subscriptions, and a wave of “optimization” branding that has learned to make a lifestyle sale look like a medical one. The demand is understandable: fatigue, low libido, weight gain, and a flattened sense of drive are real and common, and the promise of a single hormone that fixes all of them is powerful. But that same promise is exactly what makes the space dangerous. Investigations in 2025 documented online clinics prescribing testosterone after a single blood draw — sometimes to men whose levels were never even low — while federal regulators flagged direct-to-consumer sites marketing testosterone for a “male menopause” indication the FDA has never approved.

The core problem is that testosterone is a real medication with real consequences, prescribed inside a system optimized to say yes. It suppresses fertility, thickens the blood, raises blood pressure, and — per TRAVERSE — carries small but real signals for clots and atrial fibrillation. None of that is disqualifying for a man who genuinely needs treatment; all of it is reckless when the “diagnosis” was a low afternoon lab and a questionnaire, when no one checked whether weight loss or sleep would have fixed the number, when fertility was never discussed, and when no one is watching the hematocrit three months in. The Endocrine Society’s 2026 statement makes the same point in clinical language: benefits are clear for confirmed disease, weight loss comes first for obesity-related low levels, and there is no basis for screening — or treating — asymptomatic men.

That is the entire argument for an evidence-based physician, and it is not a marketing line — it is a function. A physician confirms the diagnosis with morning, repeated, properly interpreted labs instead of a single number. A physician asks whether the real driver is sleep, weight, alcohol, or medication before reaching for a hormone. A physician protects a younger man’s fertility with hCG or a SERM instead of quietly ending it, tailors the formulation to the person rather than the subscription, keeps estradiol in balance instead of crushing it, and monitors the hematocrit, PSA, and blood pressure that a checkout page ignores. Used this way, testosterone is genuinely restorative — and preserving strength, bone, metabolic health, and vitality in a man who is truly deficient is preventive medicine in the most literal sense.

So where does that leave a thoughtful man weighing this? Not cynical, and not credulous. Testosterone therapy is legitimately effective and, for the right patient, legitimately worth having; the science on its safety is better now than it has ever been. The difference between a result and a regret is almost always whether someone with medical judgment — not an algorithm optimizing for a refill — decided whether you needed it, what form to use, and what to watch. That is the whole project: not the average man in an ad, but the actual one in the room.

If You’re Considering TRT