Testosterone therapy can cause a range of side effects, from common nuisances like acne and mood shifts to more serious concerns like thickened blood and reduced fertility. The specific risks depend on dosage, how long you’ve been on treatment, and your individual health profile. Here’s what to expect and what to watch for.
Thickened Blood Is the Most Common Serious Risk
The side effect that gets the most clinical attention is polycythemia, a condition where your body produces too many red blood cells. Testosterone stimulates red blood cell production in bone marrow, which is useful in moderation but dangerous in excess. When red blood cell concentration climbs too high, your blood becomes thicker and harder to pump, raising the risk of blood clots, stroke, and heart attack.
Polycythemia is typically defined as a hematocrit level (the percentage of your blood made up of red blood cells) at or above 52%. In a large study of nearly 49,000 men on testosterone therapy, roughly 12% developed at least one hematocrit reading above that threshold. Men who crossed into polycythemia territory faced a higher risk of major cardiovascular events and venous blood clots, particularly in their first year of treatment. This is why routine blood work is non-negotiable while you’re on testosterone. Standard monitoring includes a hematocrit check at three months, six months, and then annually once levels are stable.
Effects on Fertility and Testicle Size
Taking testosterone from an outside source signals your brain to stop telling your testicles to produce their own. Your pituitary gland reduces its output of the hormones that drive both natural testosterone production and sperm production. The result is a significant drop in sperm count, sometimes to zero. For men who want to have children, this is one of the most important side effects to understand before starting treatment.
Testicle size also decreases over time. In one study tracking men over two years of therapy, average testicular volume dropped from 16.5 ml to 13.7 ml, a reduction of about 17%. Both sperm suppression and testicular shrinkage are generally reversible after stopping testosterone, but recovery can take months, and full fertility isn’t guaranteed for every man.
Breast Tissue Growth
Your body converts a portion of testosterone into estrogen through an enzyme called aromatase, which is active in fat, muscle, and skin tissue. When testosterone levels rise, estrogen levels can rise too, sometimes enough to cause breast tissue swelling or tenderness. This is more common in men who carry more body fat, since fat tissue is particularly active in converting testosterone to estrogen. The effect can range from mild puffiness to noticeable breast development, and it sometimes requires treatment adjustment or medication to manage the conversion process.
Sleep Apnea and Sleep Quality
Testosterone therapy can trigger or worsen obstructive sleep apnea, a condition where your airway repeatedly collapses during sleep. In one comparison, the two-year risk of developing sleep apnea was 16.5% among men on testosterone versus 12.7% in a control group. Research has also shown that testosterone can reduce total sleep time by roughly an hour per night in some men and increase the duration of low oxygen levels during sleep by about five minutes per night.
The connection between testosterone and sleep apnea appears to run in both directions. At least 35% of men with low testosterone who develop polycythemia also carry a sleep apnea diagnosis, suggesting that the blood-thickening effect and breathing disruption during sleep can compound each other. If you snore heavily, wake up gasping, or feel exhausted despite a full night’s sleep after starting testosterone, a sleep study is worth pursuing.
Heart and Cardiovascular Effects
The cardiovascular safety of testosterone therapy has been debated for over a decade. The most reassuring data comes from a large meta-analysis published in The Lancet Healthy Longevity, which pooled individual patient data and found no statistically significant difference in cardiovascular events between men on testosterone and those on placebo. About 7.5% of men on testosterone experienced a cardiovascular event compared to 7.2% on placebo, a gap well within the range of chance. The types of events seen in both groups included irregular heart rhythms, coronary artery problems, and heart failure at similar rates.
That said, this overall picture doesn’t eliminate risk for individual men. The polycythemia data suggests that men whose blood thickens significantly on testosterone do face elevated cardiovascular danger, particularly in the first year. The key takeaway: testosterone therapy doesn’t appear to raise heart risk across the board, but unmonitored changes in blood thickness can create real problems.
Prostate Concerns
The relationship between testosterone and prostate cancer has a complicated history. For decades, the assumption was that testosterone fuels prostate cancer growth, and product labels for every testosterone formulation still list a history of prostate cancer as a contraindication. The American Urological Association now takes a more nuanced position: there isn’t enough evidence to definitively say testosterone therapy is safe for men with a history of prostate cancer, but there also isn’t strong evidence that it causes prostate cancer in healthy men.
What’s recommended is baseline PSA testing before starting therapy for men over 40, with follow-up PSA checks at three and six months, then annually. If your baseline PSA is elevated, a second test is typically done to rule out a temporary spike before making any decisions. Men already being treated for prostate cancer who choose to go on testosterone need more frequent monitoring, and that decision should be made carefully with full awareness that the risk-benefit ratio isn’t well quantified.
Skin and Cosmetic Changes
Acne is one of the earliest and most visible side effects. It tends to peak during the first year of treatment and then gradually improve, following the same pattern as puberty-related acne. Oily skin often accompanies it. Some men also experience hair thinning or accelerated male-pattern baldness, particularly if they’re genetically predisposed. Increased body hair growth follows your family’s pattern but can take five or more years to reach its full extent.
When Side Effects Typically Appear
Side effects don’t all show up on the same schedule. Some changes happen within weeks: libido shifts, mood changes, and early voice changes (a scratchy or hoarse feeling in the throat) can appear almost immediately. Acne and oily skin tend to develop within the first few months and peak around the one-year mark.
Body composition changes, including shifts in muscle and fat distribution, take longer to settle. Surgeons who perform chest procedures related to hormonal changes often recommend waiting at least 6 to 12 months for the soft tissue contours to stabilize. Facial changes from testosterone can take two or more years to fully develop. Body hair growth is the slowest change, potentially evolving over five years or longer. The blood-thickening effect can develop at any point but is most dangerous in the first year, which is why monitoring is more frequent early on.
Routine Monitoring on Testosterone
Before starting testosterone therapy, baseline blood work should include hematocrit (red blood cell concentration) and, for men over 40, a PSA test. After starting treatment, those same tests are repeated at three months and six months. Once levels are stable and no red flags have appeared, annual monitoring is the standard. Testosterone levels themselves are checked to make sure the dose is keeping you in the normal range, not pushing you above it, since supraphysiological levels increase the likelihood of every side effect listed above.
If your hematocrit creeps above 52%, your provider will typically lower the dose or temporarily pause treatment. If sleep apnea symptoms develop, a sleep study can determine whether testosterone is contributing. And if you’re planning to have children in the near future, discussing fertility preservation before starting therapy gives you far more options than trying to recover sperm production after the fact.