Testosterone therapy carries a range of side effects, from common and manageable issues like acne and fluid retention to more serious concerns involving blood thickness, fertility, and cardiovascular health. Most men on therapy will experience at least one side effect, though severity varies widely depending on the dosage, delivery method, and individual health profile. Here’s what to expect and what actually matters.
Thicker Blood Is the Most Common Concern
The single most important side effect to understand is an increase in red blood cell production. Your body responds to higher testosterone levels by making more red blood cells, which thickens your blood. This is measured by a lab value called hematocrit, and rates of this effect run surprisingly high: up to 66.7% of men on testosterone develop elevated levels, with injectable forms, higher doses, and older age all increasing the risk.
Thicker blood raises the chance of blood clots, stroke, and other vascular problems. Current guidelines recommend reducing the dose or pausing therapy if hematocrit reaches 54%, and investigating the cause if it crosses 50%. This is why regular blood work isn’t optional on testosterone therapy. Most doctors will check your levels within the first few months and periodically after that. If your numbers climb too high, a dose reduction or therapeutic blood draw can bring them back down.
Skin and Cosmetic Changes
Acne and oily skin are among the first side effects men notice, often within weeks of starting therapy. Testosterone increases oil production in the skin, which can trigger breakouts on the back, shoulders, and face. For most men this is mild and manageable with basic skincare, though some need prescription treatment.
Breast tissue growth is another recognized risk. The body converts a portion of testosterone into estrogen, and when that balance tips, breast tissue can enlarge. This can happen at any dose but is more common with higher levels. Hair changes are also possible. Men genetically predisposed to male pattern baldness may notice accelerated thinning, since testosterone’s more potent byproduct in the scalp drives hair follicle miniaturization.
Fertility Takes a Direct Hit
If you’re considering having children, this is the side effect that matters most. Testosterone therapy suppresses sperm production, sometimes dramatically. When you introduce testosterone from outside the body, your brain registers the high levels and dials back its own signals to the testes. Specifically, it reduces a hormone called FSH that’s essential for making sperm. The result can be a sperm count near zero.
This effect is reversible for most men, but recovery isn’t instant. After stopping therapy, it can take months for sperm production to return to baseline, and there’s no guarantee it will fully recover in every case. Men who want to preserve fertility should discuss this before starting treatment, not after.
Cardiovascular Risk: What the Evidence Shows
The relationship between testosterone therapy and heart health has been one of the most debated topics in men’s health. The largest trial to date, called TRAVERSE, followed over 5,100 men for an average of 22 months. Its conclusion: testosterone therapy was no worse than placebo for major cardiac events like heart attack and stroke. That’s reassuring, but it’s not the whole picture.
Earlier studies pointed in the other direction, showing more cardiovascular events and greater buildup of arterial plaque in men given testosterone. The current consensus treats the risk as neither confirmed nor fully ruled out, particularly for men who already have heart disease or significant risk factors. Your baseline cardiovascular health matters a lot when weighing this decision.
Prostate Health and PSA Levels
For years, men were told testosterone therapy could fuel prostate cancer. The current evidence doesn’t support that. Multiple studies of older men with low testosterone who received therapy found no higher rates of prostate cancer compared to men who didn’t. Testosterone itself does not appear to increase the risk of developing prostate cancer.
What does happen is that PSA levels, a marker used to screen for prostate issues, often rise after starting therapy. Higher PSA leads to more biopsies, and more biopsies detect cancers that might have otherwise gone unnoticed. This creates the appearance of increased risk when the actual biology hasn’t changed. Guidelines recommend checking PSA before starting treatment and again 3 to 12 months later. A confirmed PSA jump of more than 1.4 points above your baseline, or a total PSA above 4.0, warrants a urology referral.
Sleep Apnea Can Get Worse
Testosterone therapy is generally considered a problem for men with untreated or severe obstructive sleep apnea. The relationship is complicated: low testosterone and sleep apnea frequently coexist, and treating one can affect the other in unpredictable ways. Short-term, high-dose therapy appears more likely to worsen breathing during sleep, while longer-term, lower doses may actually improve symptoms in some men.
The Endocrine Society classifies worsening sleep apnea as an uncommon side effect with a weak link to therapy, but still recommends against starting treatment in men with untreated severe sleep apnea. If you develop snoring, daytime sleepiness, or gasping during sleep after starting testosterone, a sleep study can determine whether the therapy is contributing.
Mood and Behavioral Shifts
Mood changes cut both ways with testosterone. Low testosterone itself causes irritability, poor concentration, and depression. Therapy often improves these symptoms, which is part of why men seek it out. But artificially elevated levels can swing the pendulum too far, producing irritability, mood swings, euphoria, impaired judgment, and in some cases uncharacteristically aggressive behavior, though the aggression link is not well proven.
These psychological effects tend to correlate with dose. Men on appropriate, monitored replacement doses are less likely to experience dramatic mood shifts than those running supraphysiological levels. If you notice personality changes that feel out of character, it’s worth checking whether your blood levels are higher than intended.
Injection Site Reactions
For men using injectable testosterone, local reactions at the injection site are common but almost always mild. In clinical data for testosterone cypionate injections, 26% of patients developed redness at the site, and 4% had a broader injection site reaction. All of these were categorized as mild, meaning slight or barely noticeable. Rotating injection sites and proper technique minimize discomfort.
Topical gels and patches avoid injection-related issues entirely but introduce their own concerns, primarily the risk of transferring testosterone to partners or children through skin contact.
Fluid Retention and Other Effects
Testosterone promotes sodium and water retention, which can cause mild swelling in the ankles or feet and a slight bump in blood pressure. This is more noticeable in the first weeks of therapy and tends to stabilize. Men with existing heart failure or kidney problems are more vulnerable to this effect. Monitoring your weight and blood pressure regularly helps catch fluid retention early.
Liver concerns are largely limited to oral testosterone formulations, which are less commonly prescribed today. Injectable and topical forms bypass the liver’s first-pass metabolism and carry minimal hepatic risk.
What Monitoring Looks Like
The Endocrine Society recommends evaluation shortly after starting therapy to check whether you’re responding, experiencing side effects, and staying on the regimen correctly. Blood work typically includes hematocrit, PSA, and testosterone levels. After the first year, prostate monitoring follows standard screening guidelines based on your age and risk profile. Most men settle into a routine of blood draws every 6 to 12 months once their levels are stable, though your doctor may check more frequently if you’re on injectable forms or have risk factors for erythrocytosis.