Most discussion of testosterone replacement therapy side effects still runs on two fears that the evidence has moved past, and skips the one problem that actually interrupts treatment. The fears are heart attacks and prostate cancer. The problem is a rising haematocrit.

This page covers what the large randomised safety trial found, what it did flag, why the prostate position softened, and the cosmetic and quality-of-life effects that decide whether men stay on therapy. It is informational. The prescribing decision belongs with a physician who knows your history.

The Verdict

The cardiac fear was largely wrong; the blood-thickening problem is real and underplayed. The largest randomised safety trial in hypogonadal men with cardiovascular risk found no excess of major adverse cardiac events, but did flag atrial fibrillation, pulmonary embolism and acute kidney injury. Prostate cancer risk is not supported by current data, though active prostate cancer remains a contraindication and PSA monitoring remains standard. Erythrocytosis is the side effect that most often forces a dose reduction or a therapeutic phlebotomy. Suppressed fertility is close to universal and is the effect men most often say they were not warned about.

Testosterone replacement therapy and cardiovascular risk

The largest randomised cardiovascular safety trial of testosterone therapy did not find an increase in major adverse cardiac events. The TRAVERSE trial, published in the New England Journal of Medicine in 2023, randomised roughly 5,200 middle-aged and older men with hypogonadism and either established cardiovascular disease or high cardiovascular risk to a transdermal testosterone gel or placebo.

On the primary endpoint, a composite of cardiovascular death, non-fatal heart attack and non-fatal stroke, testosterone was not worse than placebo. That result addressed a decade of uncertainty created by earlier observational studies and a 2015 FDA label change warning of possible cardiovascular risk.

The trial did flag three things. Atrial fibrillation, pulmonary embolism and acute kidney injury each occurred more often in the testosterone group. None was the primary question the trial was built to answer, so they are signals rather than settled conclusions, and they are what a prescriber should now be discussing. The clot signal fits the erythrocytosis mechanism described below.

Can testosterone replacement therapy cause prostate cancer?

Current evidence does not support the claim that testosterone therapy causes prostate cancer. The old position traces to 1941 work showing that surgical castration made metastatic prostate cancer regress, which was generalised into the assumption that more testosterone must mean more prostate cancer.

The androgen saturation model explains why that generalisation does not hold. Prostate tissue has a finite number of androgen receptors, and they are close to fully occupied at serum testosterone concentrations near the bottom of the normal range. Raising a man from a deficient level to a normal one adds real stimulus. Raising him from normal to high adds very little, because the receptors are already taken. This is why prostate volume and PSA typically rise modestly early in therapy and then plateau, rather than climbing indefinitely with the dose.

What has not changed is the monitoring. PSA belongs in the baseline panel for men over 40, is rechecked between three and twelve months after starting, and then follows normal screening intervals. A PSA rise greater than roughly 1.4 ng/mL within a single year warrants urology input. The genuine contraindications remain active or untreated prostate cancer, a palpable prostate nodule, and an unexplained elevated baseline PSA. Any of those needs evaluating before a prescription, not after.

Erythrocytosis: the side effect that most often changes the plan

A rising haematocrit is the most frequent laboratory abnormality on testosterone therapy and the most common reason a working dose gets reduced. Testosterone stimulates erythropoiesis directly and suppresses hepcidin, which frees up more iron for red cell production. The result is more red cells, thicker blood, and a haematocrit that can climb well beyond the normal range within months.

Most prescribers act above roughly 54%. The usual sequence is a dose reduction or a longer dosing interval first, then therapeutic phlebotomy if the number stays high. A baseline haematocrit already above roughly 50% is a reason not to start until the cause is understood.

Route matters more here than for any other side effect. Injections produce a higher peak concentration than gels or creams, and the peak is what drives red cell production, so weekly or fortnightly injections raise haematocrit more than daily transdermal dosing does. Splitting the same weekly dose into two or three smaller injections often brings the number down without reducing the total dose at all. That is the first adjustment worth asking about.

Every side effect, listed alphabetically

The table below indexes the effects men actually report, with the mechanism behind each and the first adjustment a prescriber usually makes. Listed alphabetically by side effect.

Testosterone therapy side effects, listed alphabetically, with mechanism and first-line response.
Side effectHow commonWhat causes itWhat to do about it
Acne and oily skin Common in the first few months Androgens stimulate sebaceous glands, worst at peak levels Lower the peak: smaller, more frequent doses. Standard acne treatment works normally.
Anxiety and irritability Uncommon at physiological doses Peak-to-trough swings, or estradiol driven too low or too high Split the dose before changing it. Measure estradiol before assuming testosterone is the cause.
Breast tenderness or gynecomastia Uncommon Aromatisation of testosterone into estradiol Reduce the dose first. An aromatase inhibitor only against a measured estradiol, never empirically.
Erythrocytosis (rising haematocrit) The most frequent lab abnormality on therapy Testosterone stimulates red cell production and suppresses hepcidin, freeing more iron Dose reduction, longer interval, or therapeutic phlebotomy. Most prescribers act above 54%.
Fluid retention Common in the first weeks Sodium and water retention Usually settles by month two. Check blood pressure while it is happening.
Hair loss (male pattern) Only in men genetically predisposed to it Conversion to DHT by 5-alpha reductase in scalp follicles Raise it before starting. Hair already lost does not return when therapy stops.
Injection or insertion site reactions Route-specific Oil volume and needle gauge for injections; a small incision for pellets Warm the vial, use a finer draw-up technique. Pellet extrusion needs the clinic that placed them.
Sleep apnea worsening Uncommon, but consequential when present Poorly characterised; suspected effects on ventilatory control and soft tissue Screen before starting. Untreated severe apnea is a reason to delay therapy, not to add to it.
Suppressed sperm production Expected in most men LH and FSH suppression collapses intratesticular testosterone Decide before the first dose: hCG alongside, a SERM instead, or sperm banking.
Testicular shrinkage Common The same LH and FSH suppression hCG alongside testosterone maintains volume in most men.

Joint pain, mood and blood pressure: the three most misread effects

Joint pain, mood swings and a rising blood pressure are the three effects men most often blame on the testosterone itself when the cause is something adjacent to it. Each produces a symptom that looks like a dosing failure, which is why the first response to any of them is rarely to change the dose.

Testosterone replacement therapy joint pain has a specific and frequently missed cause: estradiol pushed too low. Some testosterone aromatises into estradiol, and men need estradiol for joint comfort, libido and bone density. When an aromatase inhibitor is prescribed empirically rather than against a measured level, the result is aching joints, flat mood and blunted libido, all of which then get blamed on the testosterone. Ask for a sensitive estradiol assay before anyone adds an aromatase inhibitor.

Mood effects usually track the dosing pattern rather than the level. Anxiety and irritability at physiological doses follow the peak-to-trough swing between doses, not the average concentration, which is why splitting the same weekly dose into smaller and more frequent ones resolves them for many men without changing the total at all. Breast tenderness and gynecomastia come from the opposite direction, estradiol running high, and usually respond to a dose reduction before anything else is added.

Blood pressure deserves its own plan rather than its own panic. It can rise through early fluid retention and later through the increased viscosity that comes with a higher haematocrit. Testosterone products carry labeled warnings about blood-pressure increase, and FDA revised testosterone labeling class-wide in 2025 following the TRAVERSE results, so this is a class consideration rather than a property of one formulation. Uncontrolled hypertension is a reason to treat any route cautiously and to have a blood-pressure plan in place before starting. Obstructive sleep apnea sits alongside it and matters twice over: it can worsen on therapy, and untreated apnea also lowers testosterone by itself, so treating the apnea sometimes removes the reason for the prescription entirely.

Fertility suppression

Exogenous testosterone suppresses LH and FSH, intratesticular testosterone collapses, and sperm production falls in most men within a few months, often to zero. Recovery after stopping is common but can take a year or longer and is not guaranteed, which is why the decision belongs before the first dose rather than after. TRT and fertility covers sperm banking, hCG alongside therapy, and the SERM-based alternatives that raise testosterone without suppressing the axis.

How the delivery route changes the side effects

The route changes which side effects you are most likely to get, not whether side effects happen at all. Injections drive the highest peaks, so they produce both the largest haematocrit rise and the peak-to-trough mood swings that smaller, more frequent doses solve.

Gels and creams carry a risk none of the other routes has: transference. Skin-to-skin contact can move enough testosterone to a partner or a child to cause virilization, which is why these products carry a boxed warning for secondary exposure. Covering the application site and washing hands manages it, but a household with young children is a genuine reason to pick a different route. Pellets add site-specific problems instead, mainly bruising, infection and occasional extrusion, and they cannot be dose-adjusted once inserted. TRT injections vs cream vs pellets compares the routes in full.

Contraindications: who should not start

Several conditions rule out testosterone therapy outright, and screening for them belongs before any prescription is written. The list below reflects the standard set used in Endocrine Society guidance.

  • Active breast cancer, or prostate cancer without specialist input
  • A palpable prostate nodule, or an unexplained elevated baseline PSA
  • Baseline haematocrit above roughly 50%
  • Untreated severe obstructive sleep apnea
  • Uncontrolled heart failure, or a cardiovascular event in the past three to six months
  • Known thrombophilia or a previous unprovoked venous thromboembolism
  • Severe untreated lower urinary tract symptoms
  • Wanting to conceive in the near term

See a physician promptly rather than a telehealth intake if you have visual field changes or new persistent headaches, nipple discharge, testicular pain or asymmetry, gynecomastia that appeared quickly, or a total testosterone under 150 ng/dL. Each of those raises the possibility of a pituitary or testicular problem that a testosterone prescription would mask rather than treat. Signs of low testosterone covers the diagnostic workup and what to rule out first.

What competent monitoring looks like

A program running testosterone therapy properly checks a defined set on a defined schedule, and the absence of that schedule is the clearest signal that a clinic is selling a subscription rather than running a protocol.

  • Testosterone level at 6 to 12 weeks after starting or changing a dose, drawn at a consistent point in the dosing cycle
  • Haematocrit at baseline, at 3 to 6 months, then at least annually for as long as therapy continues
  • PSA at baseline in men over 40, again at 3 to 12 months, then on normal screening intervals
  • Blood pressure at every review, and more often in the first three months
  • Sensitive estradiol when there are symptoms, and always before an aromatase inhibitor is added
  • Ferritin once therapeutic phlebotomy becomes routine
  • A symptom review that asks specifically about sleep, mood and libido, not just the numbers

TRT clinics compared applies the same rubric to the main physician-led programs, and doctor-led options lists the supervised routes for getting this monitoring in place.

TRT pros and cons, weighed together

A page that lists only the downsides of testosterone replacement therapy misinforms as badly as a marketing page that lists only the benefits. In men with confirmed hypogonadism, therapy reliably improves sexual function, bone density, lean mass, and often mood and energy. Those are the reasons it is prescribed, and they are real.

The negatives are equally concrete. It is a long-term commitment with a monitoring schedule attached, it suppresses your own production, it suppresses fertility, and it requires ongoing blood work you cannot skip. The useful question is not whether testosterone replacement therapy is dangerous in the abstract. It is whether your diagnosis is confirmed, whether the reversible causes were ruled out, and whether you have a prescriber who will actually run the schedule above.

Frequently Asked Questions

Is testosterone replacement therapy safe?

For a man with confirmed low testosterone and no contraindication, the safety record is better than its reputation. TRAVERSE randomised roughly 5,200 men with hypogonadism and cardiovascular risk and found no increase in major adverse cardiac events, while flagging atrial fibrillation, pulmonary embolism and acute kidney injury. The larger safety variable is whether the diagnosis is real. Testosterone given to a man whose fatigue actually comes from untreated sleep apnea carries the risk with none of the benefit.

What is the difference between TRT and anabolic steroids?

The difference is the dose and the target, not the molecule. Replacement aims to put a man with a diagnosed deficiency back inside the normal range and hold him there under monitoring, usually on one testosterone ester. Anabolic steroid use for physique or performance aims well above the physiological range, frequently stacking several compounds at once, and that supraphysiological territory is where the severe cardiac, liver and psychiatric harms cluster. Both suppress your own production, so testicular shrinkage and lost fertility are shared consequences rather than distinguishing ones. The practical test is whether the dose is aimed at a lab range or at a physique.

Can testosterone replacement therapy cause blood clots?

Yes, and it is the clearest safety signal in the modern data. TRAVERSE reported more pulmonary embolism on testosterone than on placebo. Two mechanisms plausibly contribute: thicker blood as haematocrit climbs, and direct effects on clotting factors. A previous unprovoked clot or a known thrombophilia belongs in the decision before the first prescription, and any sudden breathlessness or calf swelling on therapy is an emergency rather than a follow-up question.

What should I expect in the first month on TRT compared with the sixth?

The subjective changes arrive first and the structural ones take months. Libido and morning erections tend to move earliest, often within the first few weeks, alongside the fluid retention that shows up on the scale before anything has changed underneath it. Mood and energy are less predictable early and are easily confused with the lift that comes from finally acting on a problem. Body composition moves on a scale of months rather than weeks, and bone density is slower still. This is why a dose is not judged on how the first month felt, and why a prescriber will usually hold a dose steady long enough to read it properly.

Does testosterone interact with other common medications?

A handful of interactions are worth raising before the first prescription. Testosterone can potentiate warfarin, so anticoagulated men need closer INR checks after starting or changing a dose. Corticosteroids add to the fluid retention. Insulin and other glucose-lowering drugs sometimes need reducing as insulin sensitivity improves. The more common problem is not a true interaction but overlapping causes: opioids, heavy alcohol and some psychiatric medications affect the hormone axis, libido or mood in their own right, and starting testosterone without accounting for them makes the result impossible to interpret. Give the prescriber the whole list, supplements included.

Can testosterone replacement therapy cause erectile dysfunction?

Testosterone therapy does not typically cause erectile dysfunction, and in men who are genuinely deficient it usually improves erections and libido. When erections stay poor despite a normal level, the usual explanation is that testosterone was never the cause. Erectile dysfunction is more often vascular, and it can be an early visible sign of the same arterial disease that shows up later in the heart, which makes it a reason for a cardiovascular assessment rather than a dose increase. Diabetes, blood pressure medication, antidepressants and anxiety about performance itself are the other frequent contributors, and adding testosterone on top of any of them does not address them.

What happens if I miss a dose or inject late?

A single missed dose is not an emergency and is not a reason to double up. Long-acting injectable esters clear over days, so a man on a weekly or fortnightly schedule usually notices a dip in energy, mood or libido toward the end of the gap rather than anything abrupt, while gels and creams fall away faster because the level tracks the most recent application closely. The standard approach is to take the dose when you remember and return to the normal schedule, since stacking two doses together recreates the peak-to-trough swing that produces most of the symptoms men blame on the drug. Tell the prescriber if doses are being missed regularly, because a level drawn against a schedule you are not keeping gets misread as needing more.

What are the long term effects of testosterone replacement therapy?

The main one is dependence on the prescription: exogenous testosterone suppresses the pituitary signal driving your own production, and that axis does not always recover quickly after stopping. Sperm production is suppressed in most men, and recovery can take a year or more. Haematocrit needs monitoring for as long as therapy continues, not only in the first year. On the benefit side, bone density, lean mass and sexual function improve and hold while treatment continues.

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