Testosterone therapy carries a well-documented set of side effects that range from cosmetically annoying to medically serious. The most common is a rise in red blood cell concentration, which affects roughly one in nine men on standard replacement doses. Other frequently reported effects include acne, a modest bump in prostate-specific antigen (PSA), reduced sperm production, and fluid retention. The cardiovascular picture, long the source of the loudest headlines, is more reassuring than it once appeared, but still has nuances worth understanding.
The Red Blood Cell Problem
The single most predictable laboratory change during testosterone therapy is a rise in hematocrit, the percentage of your blood volume occupied by red blood cells. Testosterone stimulates the kidneys to produce more erythropoietin, which in turn tells bone marrow to churn out more red cells. A multi-institutional study found that about 11% of men on testosterone replacement developed polycythemia, defined as a hematocrit at or above 52%, compared with under 2% of men treated with an alternative medication for low testosterone.1PubMed. A Comparison of Secondary Polycythemia in Hypogonadal Men Treated with Clomiphene Citrate versus Testosterone Replacement: A Multi-Institutional Study On average, hematocrit climbs by about three percentage points during treatment.
Why does this matter? Thicker blood flows less easily. A large database study involving nearly 49,000 men found that those who developed polycythemia while on testosterone therapy faced a higher risk of major cardiovascular events and venous blood clots in the first year of treatment.2PubMed Central. Secondary Polycythemia in Men Receiving Testosterone Therapy Increases Risk of Major Adverse Cardiovascular Events and Venous Thromboembolism in the First Year of Therapy Testosterone therapy may raise the risk of venous thromboembolism partly through this mechanism of elevated hematocrit and increased blood viscosity.3JAMA Internal Medicine. Association of Testosterone Therapy With Risk of Venous Thromboembolism Among Men With and Without Hypogonadism This is why most prescribing guidelines call for a blood count before starting therapy and periodic checks afterward, typically every six to twelve months. If hematocrit creeps too high, the usual response is to lower the dose, switch the delivery method, or temporarily pause treatment.
Cardiovascular Safety
For years, the biggest worry about testosterone therapy was heart attacks and strokes. Early observational studies in the 2010s triggered FDA safety advisories, and the resulting fear kept many men and their doctors away from treatment even when it was clearly indicated. The evidence was messy, though. Reviews of the early data found that observational and randomized studies had produced conflicting results, and meta-analyses were inconclusive, reflecting huge differences in study design.4PubMed Central. The Effect of Testosterone on Cardiovascular Disease and Cardiovascular Risk Factors in Men: A Review of Clinical and Preclinical Data
The question was finally addressed head-on by the TRAVERSE trial, the largest randomized, placebo-controlled cardiovascular safety study of testosterone therapy to date. It enrolled over 5,000 middle-aged and older men who already had cardiovascular disease or were at high risk. After a mean follow-up of roughly three years, major adverse cardiovascular events occurred in about 7% of men in both the testosterone group and the placebo group, with no statistically significant difference between them.5PubMed. Cardiovascular Safety of Testosterone-Replacement Therapy Rates of heart attack, stroke, cardiovascular death, and heart failure were also similar between groups. A European expert panel reviewing these results concluded that the trial provides robust evidence that testosterone therapy, when correctly administered, does not increase the risk of major cardiovascular events.6PubMed Central. Cardiovascular safety of testosterone therapy—Insights from the TRAVERSE trial and beyond: A position statement of the European Expert Panel for Testosterone Research
That same panel did note a small but significant increase in atrial fibrillation and non-fatal arrhythmias in the testosterone-treated group. This finding has not received as much attention as the main cardiovascular results, but it is worth flagging for men who already have a history of irregular heart rhythms. Lipid profiles improved modestly in the testosterone group, with higher HDL cholesterol, while blood pressure and blood sugar markers remained stable overall.6PubMed Central. Cardiovascular safety of testosterone therapy—Insights from the TRAVERSE trial and beyond: A position statement of the European Expert Panel for Testosterone Research
What Testosterone Does to Cholesterol
The lipid story is a bit contradictory depending on which evidence you look at. A meta-analysis of body composition and lipid studies found that testosterone therapy lowered total cholesterol, with no change in LDL, but it also lowered HDL cholesterol, the so-called “good” cholesterol, particularly in men who started with higher testosterone levels at baseline.7PubMed. Effects of testosterone on body composition, bone metabolism and serum lipid profile in middle-aged men: a meta-analysis A review of the broader literature confirmed this pattern: meta-analyses have generally shown that exogenous testosterone lowers HDL, though it tends to bring down total cholesterol and LDL at the same time.8PubMed Central. An update on testosterone, HDL and cardiovascular risk in men Other studies have found no significant lipid changes at all after three months of treatment.9Journal of Lipid Research. Testosterone replacement in hypogonadal men alters the HDL proteome but not HDL cholesterol efflux capacity
In practical terms, the cholesterol shifts are usually modest and unlikely to be the deciding factor for most men. But if you already have borderline lipid numbers, it is one more thing for your doctor to track.
Blood Pressure
Blood pressure effects of testosterone therapy have often been described as mild or absent in typical populations, and the TRAVERSE trial found that blood pressure remained stable overall. However, a randomized placebo-controlled trial in men with opioid-induced testosterone deficiency found a more pronounced effect: systolic blood pressure rose by about 6 mmHg in the testosterone group while it dropped by about 7 mmHg in the placebo group, yielding a meaningful difference between groups.10Journal of Hypertension. Blood pressure responses to testosterone therapy are amplified by hematocrit levels in opioid-induced androgen deficiency: a double-blind, randomized, placebo-controlled trial The increase tracked with hematocrit, suggesting the same red-blood-cell thickening drives part of the blood pressure response. Men with obesity or hematocrit levels already near the upper end of normal appeared most vulnerable. If you are starting testosterone and you already run on the high side for blood pressure, this is worth monitoring closely.
Prostate and PSA
Testosterone does not appear to cause prostate cancer, but it does affect prostate tissue in ways that need watching. The TRAVERSE trial’s prostate analysis found no significant difference in rates of high-grade or any-grade prostate cancer between men on testosterone and those on placebo after more than 14,000 person-years of follow-up.11JAMA Network Open. Prostate Safety Events During Testosterone Replacement Therapy in Men With Hypogonadism: A Randomized Clinical Trial Rates of acute urinary retention, prostate biopsy, and surgical procedures were also similar between groups.
What testosterone therapy does do is nudge PSA levels upward, particularly in the first year. The TRAVERSE data showed the biggest bump at around 12 months, after which the difference between treated and untreated men shrank and was no longer significant.11JAMA Network Open. Prostate Safety Events During Testosterone Replacement Therapy in Men With Hypogonadism: A Randomized Clinical Trial A separate meta-analysis confirmed a small overall PSA increase with testosterone treatment, particularly with injectable formulations, though the increase was described as minimal and the odds of PSA elevations above threshold were no different between treated and untreated men.12PubMed Central. The effect of testosterone replacement therapy on prostate-specific antigen (PSA) levels in men being treated for hypogonadism: a systematic review and meta-analysis The practical takeaway: a slight PSA rise after starting testosterone is expected and does not by itself mean anything is wrong, but it can complicate the interpretation of a PSA screening test. If your PSA rises sharply or continuously, that warrants further evaluation regardless of whether you are on testosterone.13PubMed Central. Rising PSA during Testosterone Replacement Therapy
Fertility and Testicular Size
This is the side effect that blindsides men most often, especially younger ones. Testosterone therapy suppresses the brain’s signals to the testes. When you add testosterone from outside the body, the hypothalamus senses adequate hormone levels and dials back production of the hormones that tell the testes to make sperm and their own testosterone.14PubMed Central. Testosterone Is a Contraceptive and Should Not Be Used in Men Who Desire Fertility Sperm counts can drop to zero in many men, effectively making testosterone therapy an unreliable but powerful contraceptive. For men who want children now or in the near future, testosterone replacement is generally a poor choice, and alternative treatments exist that raise testosterone levels without shutting down sperm production.
The testes also physically shrink. A study tracking testicular volume over two years of treatment found that the average volume dropped from about 16.5 mL before treatment to 13.7 mL at 24 months, with the shrinkage becoming significant from six months onward.15The Journal of Sexual Medicine. Characteristics of testicular atrophy during testosterone replacement therapy (TRT) Both sperm suppression and testicular atrophy are generally reversible after stopping therapy, but recovery can be slow. One study of men stopping long-acting testosterone injections after two years found that full reproductive hormone recovery took up to 15 months.16European Journal of Endocrinology. Recovery of male reproductive endocrine function after ceasing prolonged testosterone undecanoate injections
Skin Changes and Site Reactions
Acne is one of the most commonly reported side effects of testosterone therapy. It makes sense physiologically: testosterone increases the activity of sebaceous glands in the skin, which can lead to oilier skin and breakouts.17International Braz J Urol. Management of Adverse Effects in Testosterone Replacement Therapy The acne is typically mild and concentrated on the back, shoulders, and face. It tends to be most noticeable in the first few months and can often be managed with standard skincare approaches.
For men using transdermal patches, skin irritation at the application site is a separate issue. An older study comparing scrotal and non-scrotal patch systems found that moderately intense skin irritation occurred at the application site in about a third of men using non-scrotal patches, compared with only 5% with scrotal patches. Allergic contact dermatitis developed in 12% of non-scrotal patch users.18PubMed. Allergy and topical irritation associated with transdermal testosterone administration: a comparison of scrotal and nonscrotal transdermal systems Modern gel formulations tend to cause less irritation than the older patch systems, which is one reason gels have largely replaced patches as the most popular topical option.
Sleep Apnea
Testosterone therapy can worsen obstructive sleep apnea in some men, and guidelines recommend screening for sleep apnea symptoms before and during treatment. Severe untreated sleep apnea is generally considered a reason to hold off on testosterone therapy until the breathing problem is addressed.19PubMed Central. Obstructive Sleep Apnea and Testosterone Deficiency The mechanism likely involves testosterone’s effects on upper airway muscle tone and fat distribution in the neck area.
There is a dose-dependent wrinkle here. High-dose testosterone appears to induce or worsen sleep apnea more reliably, while more typical replacement doses may not, and the effect may fade with longer-term therapy.20PubMed Central. Sleep, testosterone and cortisol balance, and ageing men If you develop new snoring, daytime sleepiness, or your partner notices pauses in your breathing after starting therapy, those symptoms deserve prompt attention.
Mood and Aggression
The popular image of testosterone-fueled rage does not hold up particularly well under controlled study conditions. In men with genuinely low testosterone, replacement therapy consistently improves mood. A clinical research center study found that treatment led to significant decreases in anger, irritability, sadness, tiredness, and nervousness, alongside improvements in energy, friendliness, and overall sense of well-being.21The Journal of Clinical Endocrinology & Metabolism. Testosterone replacement therapy improves mood in hypogonadal men–a clinical research center study Another study specifically examining aggression in both low-testosterone and normal-testosterone men found no increase in self- or partner-reported aggression at supraphysiological doses within a moderate range. In the hypogonadal group, treatment actually reduced tension, anger, and fatigue while boosting vigor.22PubMed. Exogenous testosterone, aggression, and mood in eugonadal and hypogonadal men
One crossover study did find a short-lived increase in anger-hostility scores at the two-week mark of treatment in young men, but importantly, that did not translate into increased aggressive behavior, and the overall psychological effects were described as limited.23The Journal of Clinical Endocrinology & Metabolism. Effects of Testosterone on Mood, Aggression, and Sexual Behavior in Young Men: A Double-Blind, Placebo-Controlled, Cross-Over Study The picture that emerges: testosterone therapy at therapeutic or moderately elevated doses improves mood more than it destabilizes it, especially in men who are starting from below-normal levels. The “roid rage” stereotype comes from anabolic steroid abuse at doses many times higher than medical replacement.
How the Delivery Method Changes the Side Effect Profile
Not all testosterone formulations carry the same risks. Injectable testosterone tends to produce higher peak levels and more dramatic troughs between doses, and this roller-coaster pattern drives certain side effects harder. A comparison study found that elevated hematocrit (above 50%) occurred in about two-thirds of men on injectable testosterone, compared with about 13% on gels and 35% on pellets.24PubMed Central. Comparison of the Effects of Testosterone Gels, Injections, and Pellets on Serum Hormones, Erythrocytosis, Lipids, and Prostate-Specific Antigen The PSA meta-analysis likewise found that intramuscular injections drove a larger PSA increase than other routes, even though the absolute rise was small.12PubMed Central. The effect of testosterone replacement therapy on prostate-specific antigen (PSA) levels in men being treated for hypogonadism: a systematic review and meta-analysis
Gels and creams produce more stable day-to-day levels and carry a lower polycythemia risk, but they introduce the possibility of transfer to household contacts through skin-to-skin contact and require daily application. Newer oral formulations that rely on lymphatic absorption rather than first-pass liver metabolism have addressed a long-standing concern about liver toxicity with older oral androgens. These formulations have shown no evidence of liver dysfunction.25PubMed Central. Testosterone Replacement Therapy: A Narrative Review with a Focus on New Oral Formulations The choice of formulation is one of the most practical levers for managing side effects, and switching from injections to a topical or oral preparation is a common first step when hematocrit or other markers run high.
Gynecomastia and Fluid Retention
Some testosterone is converted to estrogen by an enzyme called aromatase, which is especially active in fat tissue. When estrogen levels rise relative to the remaining androgen balance, breast tissue can enlarge, a condition called gynecomastia.26PubMed Central. Gynecomastia: Clinical evaluation and management This is more common in men with higher body fat and in those using higher doses. Fluid retention, meaning mild swelling in the ankles or feet and a slight bump in weight from water, is another recognized side effect linked to the same hormonal shifts.13PubMed Central. Rising PSA during Testosterone Replacement Therapy Both effects are usually mild and can often be managed by adjusting the testosterone dose or, in the case of gynecomastia, adding a medication that blocks aromatase activity.
Side Effects in Women
Testosterone therapy is prescribed to some women, typically at much lower doses than men receive, most often for low sexual desire after menopause. At physiological female doses, side effects are usually mild and reversible, primarily acne and increased facial or body hair. But if dosing creeps above physiological levels, virilizing effects can occur, including voice deepening and clitoral enlargement.27PubMed Central. Should we be prescribing testosterone to perimenopausal and menopausal women? A guide to prescribing testosterone for women in primary care Some of these changes, particularly voice changes, may not fully reverse after stopping treatment. Monitoring blood testosterone levels at baseline, at three months, and then annually is the standard approach to keeping doses in a safe range.
What Happens When You Stop
Stopping testosterone therapy is not like stopping a vitamin. Because the body’s own production has been suppressed during treatment, there is a transition period where testosterone levels drop back to their pre-treatment low, and sometimes temporarily even lower. A study that tracked men who interrupted and then resumed therapy found that the interruption led to worsening of obesity markers, urinary symptoms, erectile function, and psychological scores, all of which reversed when treatment was restarted.28PubMed. Effects of testosterone replacement therapy withdrawal and re-treatment in hypogonadal elderly men upon obesity, voiding function and prostate safety parameters Reproductive hormone recovery after stopping long-acting injections takes roughly 15 months to complete, and persistent effects on certain liver-produced proteins suggest the body carries traces of exogenous testosterone influence for a while beyond that.16European Journal of Endocrinology. Recovery of male reproductive endocrine function after ceasing prolonged testosterone undecanoate injections
This slow recovery is one reason many clinicians describe hypogonadism as a condition that may require lifelong treatment once started. It is also why abruptly stopping therapy without medical guidance is discouraged: the rebound low can feel worse than the original deficiency because the body has adapted to the exogenous supply.
Off-Label Use and the Misuse Problem
A significant share of testosterone prescriptions are written for men who do not meet established diagnostic criteria for hypogonadism. Some clinics and websites promote testosterone as a cure-all for fatigue, aging, and low motivation, leading people to believe their symptoms are attributable to a hormone deficiency and that treatment will eliminate them.29Endocrine Practice. AACE Position Statement Off-Label Use and Misuse of Testosterone, Growth Hormone, Thyroid Hormone, and Adrenal Supplements: Risks and Costs of a Growing Problem When men with normal testosterone levels take replacement doses, they assume the same side-effect risks described throughout this article but without a clear clinical benefit to offset them. The risk-benefit equation only makes sense when there is an actual deficiency to correct, confirmed with repeated blood tests and accompanied by consistent symptoms.