Taking testosterone under medical supervision for a diagnosed deficiency is generally not bad for you, and for many men it significantly improves quality of life. But it does carry real risks that depend on your health profile, the form you take, and how closely you’re monitored. The largest safety trial to date, called TRAVERSE, found that men on testosterone therapy had no higher rate of heart attacks or strokes than men on placebo. That said, testosterone can raise your red blood cell count to dangerous levels, suppress your fertility, and worsen sleep apnea. Whether the trade-off makes sense depends entirely on why you’re taking it and how it’s managed.
Who Actually Needs Testosterone Therapy
Testosterone replacement is intended for men with a clinical diagnosis of testosterone deficiency, sometimes called hypogonadism. The American Urological Association defines this as a total testosterone level below 300 ng/dL, confirmed by two separate blood draws taken in the early morning when levels naturally peak. The number alone isn’t enough for a diagnosis. You also need to have symptoms: low sex drive, erectile problems, persistent fatigue, loss of muscle mass, depressive mood, poor concentration, or reduced motivation.
If your levels are low but you feel fine, or if your levels are normal but you’re hoping for a performance boost, the risk-benefit math shifts considerably. Most of the safety data we have applies to men who genuinely need the therapy. Using testosterone when your body already makes enough introduces risks without the same payoff, and at supraphysiological doses (the kind used in bodybuilding), the dangers multiply.
Heart Attack and Stroke Risk
For years, the biggest concern about testosterone was cardiovascular safety. The TRAVERSE trial, published in 2023 with over 5,000 men, was designed specifically to answer this question. Men on testosterone experienced major cardiovascular events (heart attack, stroke, or cardiovascular death) at a rate of 7.0%, compared to 7.3% in the placebo group. The hazard ratio was 0.96, meaning no meaningful difference. This was strong enough to confirm that testosterone therapy, when used as indicated, does not increase the risk of heart attacks or strokes.
One cardiovascular concern did emerge from the same trial: pulmonary embolism, a blood clot in the lungs, occurred in 0.9% of men on testosterone versus 0.5% on placebo. That’s a small absolute difference, but it’s statistically significant and connects to testosterone’s broader effect on blood clotting, which is worth understanding on its own.
Blood Thickening Is the Most Common Risk
The single most frequently monitored side effect of testosterone therapy is erythrocytosis, an increase in red blood cells that makes your blood thicker and more viscous. This raises the risk of blood clots, which can lead to deep vein thrombosis, pulmonary embolism, or stroke. Doctors track this through your hematocrit level, the percentage of your blood volume occupied by red blood cells.
The Endocrine Society recommends stopping testosterone if hematocrit rises above 54%, then restarting at a lower dose once it normalizes. The American Urological Association is slightly more cautious, recommending investigation when hematocrit exceeds 50% and dose reduction or discontinuation at 54%. Some clinicians refer patients for a therapeutic blood draw (phlebotomy) at even lower thresholds. This is why routine blood work every few months is non-negotiable during testosterone therapy. Caught early, erythrocytosis is manageable. Ignored, it becomes dangerous.
Fertility Can Be Severely Affected
This is the risk that catches many younger men off guard. When you take external testosterone, your brain detects the higher levels and stops sending the hormonal signals that tell your testicles to produce sperm. The result is severe suppression of sperm production, often to the point of zero sperm in the ejaculate. Your testicles may also shrink noticeably.
If you stop testosterone therapy, sperm production can recover, but the timeline is unpredictable. Without any additional treatment, recovery can take up to two years. With medical intervention using medications that stimulate the brain’s hormonal signals, recovery typically begins around four to five months. Some men never fully recover their baseline fertility. If you’re planning to have children, this is one of the most important factors to discuss before starting therapy.
Prostate Cancer Fears Are Largely Unfounded
The idea that testosterone fuels prostate cancer has lingered since the 1940s, but modern evidence doesn’t support it. Multiple studies of men with low testosterone who were treated with replacement therapy have found no higher rates of prostate cancer compared to untreated men. The Mayo Clinic’s current position is that evidence does not support a link between testosterone therapy and the development of new prostate cancer.
There is a nuance worth knowing. Testosterone therapy can raise PSA levels, the blood marker used to screen for prostate cancer. Higher PSA levels lead to more biopsies, and more biopsies detect more cancers that might have gone unnoticed otherwise. This means testosterone may increase the chance of a prostate cancer diagnosis without actually causing the cancer. It’s a detection effect, not a causation effect. Your doctor will monitor PSA levels regularly during treatment.
Liver Damage Depends on the Formulation
Old oral forms of testosterone, particularly methyltestosterone developed in the 1930s, were genuinely toxic to the liver. They caused abnormal liver function tests, bile flow problems, and jaundice. This gave testosterone a lasting reputation for liver harm.
Modern formulations have largely solved this problem. Injectable testosterone (cypionate and enanthate) bypasses the liver entirely, entering the bloodstream directly from the muscle. Newer oral formulations use a different chemical structure that gets absorbed through the lymphatic system in your gut rather than passing through the liver first. Clinical trials of these newer oral versions have shown no clinically significant liver toxicity and, in some cases, no increase in liver enzyme levels at all. If you’re using a current, prescribed form of testosterone, liver damage is not a significant concern.
Sleep Apnea Can Get Worse
Testosterone therapy can worsen obstructive sleep apnea or, in some cases, trigger it for the first time. A large comparison found that the two-year risk of developing sleep apnea was 16.5% in men on testosterone versus 12.7% in controls. The effect appears to be most pronounced in the early weeks of treatment and in men who are already obese or have severe sleep apnea.
One study found that testosterone worsened nighttime oxygen levels at seven weeks, but this effect disappeared by 18 weeks, suggesting the body may partially adapt. Still, if you snore heavily, wake up gasping, or feel exhausted despite a full night’s sleep, these symptoms deserve attention before and during testosterone therapy. Untreated sleep apnea carries its own serious cardiovascular risks.
Estrogen-Related Side Effects
Your body naturally converts some testosterone into estrogen through a process called aromatization. When you add external testosterone, estrogen levels can rise enough to cause breast tenderness or breast tissue growth (gynecomastia), water retention, and mood changes. Men with more body fat tend to convert more testosterone to estrogen, since fat tissue is where much of this conversion happens.
Doctors typically monitor estrogen levels during therapy and intervene when they climb above certain thresholds, particularly if you’re experiencing symptoms. This is manageable with dose adjustments or additional medication, but it’s another reason why testosterone therapy requires ongoing monitoring rather than a set-it-and-forget-it approach.
The Bottom Line on Risk
Testosterone therapy for a genuine deficiency, prescribed at appropriate doses and monitored with regular blood work, is not inherently dangerous. The cardiovascular fears that dominated headlines for years have not been confirmed by the best available evidence. The real risks are specific and manageable: blood thickening, fertility suppression, worsened sleep apnea, and estrogen-related side effects. Each of these can be caught early with routine monitoring and addressed through dose changes or complementary treatments.
The picture changes dramatically when testosterone is used without medical oversight, at doses far above replacement levels, or by people who don’t actually have a deficiency. In those scenarios, the risks are higher and less predictable. The safety data we have is built around supervised, indicated use, and that’s where the reassuring numbers apply.