For most men with clinically low testosterone, replacement therapy is considered safe in the short to medium term when properly monitored. A large meta-analysis published in The Lancet Healthy Longevity found no increased risk of heart attack, stroke, or other cardiovascular events during treatment. That said, “safe” comes with important caveats: certain pre-existing conditions can make therapy risky, regular blood work is essential, and there are real side effects that range from manageable to serious depending on your health profile.
What Counts as Low Testosterone
The American Urological Association defines low testosterone as a total level below 300 ng/dL. To confirm the diagnosis, you need two separate blood draws, both taken in the early morning when testosterone naturally peaks. This two-test requirement exists because testosterone fluctuates day to day based on sleep, stress, illness, and even the time you ate. A single low reading isn’t enough to start treatment.
Symptoms alone don’t qualify someone for therapy either. Fatigue, low libido, and mood changes overlap with dozens of other conditions. The combination of confirmed low levels plus symptoms is what points toward a genuine deficiency worth treating.
Heart Health and Testosterone
Cardiovascular risk was the biggest safety concern for years, fueled by a few early studies that suggested testosterone might increase heart attacks and strokes. The picture has shifted considerably. A meta-analysis combining individual patient data from multiple randomized trials found that testosterone treatment is not associated with increased risk of any cardiovascular event subtype in the short to medium term. Rates of hypertension, blood clots, and stroke-related events were similar between men receiving testosterone and those on placebo.
That doesn’t mean cardiac risk is zero. Men who have had a heart attack or stroke within the past six months are advised against starting therapy. The same applies to men with uncontrolled heart failure. For men without recent cardiac events, the current evidence is reassuring, though most studies have followed patients for only a few years rather than decades.
Blood Clot Risk in the First Six Months
One risk that does show up clearly in the data is a temporary increase in blood clots. A large population-based study published in The BMJ found that during the first six months of treatment, the rate of venous thromboembolism (deep vein clots or pulmonary embolisms) was about 63% higher than in men not taking testosterone. In practical terms, that translates to roughly 10 additional clot events per 10,000 men per year above the baseline rate of about 16 per 10,000.
After six months, the elevated risk disappeared entirely. Men who had been on therapy for longer than six months showed no difference in clot rates compared to untreated men. This early-window pattern is why blood monitoring matters most in the first year. Men with a personal or family history of clotting disorders are generally advised against starting therapy altogether.
Prostate Cancer Concerns
The longstanding fear that testosterone fuels prostate cancer dates back to research from the 1940s. Modern evidence tells a different story. According to Mayo Clinic’s review of the literature, current evidence does not support a link between testosterone replacement and the development of new prostate cancer. Studies of older men with low testosterone who received treatment did not show higher rates of prostate cancer compared to untreated men.
Even among men who were previously treated for prostate cancer and showed no signs of recurrence, testosterone therapy doesn’t appear to increase the risk of the cancer returning. Still, men with active breast or prostate cancer, a palpable prostate nodule, or an elevated PSA level above 4 ng/dL are advised not to use testosterone. The guideline isn’t based on strong evidence of harm so much as an abundance of caution in a setting where the stakes are high.
Fertility and Sperm Production
This is one of the most underappreciated risks of testosterone therapy. Adding testosterone from an outside source signals your brain to stop telling the testes to produce their own. The result: sperm production drops dramatically. Most men on therapy develop very low sperm counts or no detectable sperm at all.
The good news is that the effect is typically reversible. After stopping testosterone, sperm generally returns to the semen within about three months, and no permanent damage is seen in most cases. But if you’re planning to have children in the near term, testosterone replacement is not recommended. Alternative treatments exist that can raise testosterone while preserving fertility, so this is a conversation worth having before starting any regimen.
Thickening of the Blood
Testosterone stimulates your bone marrow to produce more red blood cells. For someone with mild anemia, this can actually be a benefit. But when red blood cell production goes too far, blood becomes thicker and harder to pump, raising the risk of clots, stroke, and other complications. This condition, called erythrocytosis, is the most common lab abnormality seen with testosterone therapy.
The American Urological Association recommends keeping hematocrit (the percentage of your blood made up of red blood cells) below 54%. Levels above that threshold typically require a dose reduction, a temporary pause in treatment, or occasionally a therapeutic blood draw to bring levels down. Hematocrit should be checked every 6 to 12 months, and more often if previous values were trending upward. This is the single most important lab value to track during treatment.
Sleep Apnea Can Worsen
If you already have obstructive sleep apnea, testosterone therapy can make it worse. Research shows that testosterone can alter how the brain’s breathing sensors respond during sleep, reduce total sleep time by roughly an hour, and increase the duration of low-oxygen episodes by about five minutes per night. It may also change the muscle tone of the upper airway, making it more prone to collapse during sleep.
In one study, oxygen levels dropped significantly at seven weeks of treatment, though the effect seemed to level off by 18 weeks. Men with severe, untreated sleep apnea are advised against starting testosterone. If you have mild or moderate sleep apnea that’s being managed with a CPAP machine, therapy may still be an option, but closer monitoring of your sleep symptoms is important.
Liver Safety With Modern Formulations
Older oral testosterone pills (specifically methyltestosterone) were known to cause significant liver damage, which gave testosterone therapy a lasting reputation for liver toxicity. Modern formulations don’t carry the same risk. A two-year study of the current FDA-approved oral testosterone found no evidence of liver toxicity while keeping testosterone levels in the normal range. Injectable, topical, and pellet forms of testosterone bypass the liver almost entirely and have never been strongly linked to liver problems.
Who Should Not Use Testosterone
The Endocrine Society maintains a clear list of situations where testosterone therapy is not recommended:
- Active prostate or breast cancer
- A recent heart attack or stroke (within the last six months)
- Uncontrolled heart failure
- Severe untreated sleep apnea
- A known clotting disorder
- Already elevated hematocrit
- Plans for fathering children in the near future
- Elevated PSA without further urological workup
The Society also recommends against routinely prescribing testosterone to all men over 65 with low levels, noting that age-related decline alone isn’t necessarily a reason to treat. And for men with type 2 diabetes and low testosterone, therapy is not recommended as a tool for improving blood sugar control.
What Monitoring Looks Like
Starting testosterone isn’t a “set it and forget it” situation. In the first year, you can expect blood draws at roughly three, six, and twelve months. These check your testosterone level to make sure the dose is working, along with a complete blood count to catch rising hematocrit early. After the first year, most guidelines call for lab work every 6 to 12 months as long as values remain stable.
Your provider will also monitor for symptoms: changes in mood, energy, sexual function, urinary patterns, and sleep quality. If hematocrit creeps above 54%, if PSA rises unexpectedly, or if new symptoms appear, the dose gets adjusted or therapy gets paused. The safety of testosterone replacement depends heavily on this ongoing monitoring. Men who obtain testosterone without medical supervision and skip blood work are taking on significantly more risk than the clinical data reflects.