Testosterone replacement therapy (TRT) carries a range of side effects, from common and manageable issues like acne and elevated red blood cell counts to more serious concerns involving fertility, sleep, and prostate health. Most side effects are predictable, dose-dependent, and reversible with monitoring or adjustment. Here’s what to expect across the major categories.
Red Blood Cell Overproduction
The most consistently monitored side effect of TRT is a condition called polycythemia, where your body produces too many red blood cells. Testosterone stimulates the bone marrow, thickening the blood and raising a value called hematocrit, which measures the percentage of your blood made up of red cells. A hematocrit above 54% is the threshold where most guidelines recommend stopping therapy or removing blood through a procedure similar to a blood donation.
This matters because thicker blood flows less easily and raises the risk of clots, stroke, and other vascular problems. American guidelines are more conservative, cautioning against starting TRT if your hematocrit is already above 50%. European guidelines set the concern threshold at 54%. Either way, this is the single lab value your doctor will watch most closely, typically checked at baseline, again at 3 to 6 months, and then annually.
Near-Complete Suppression of Sperm Production
TRT effectively acts as male birth control. When you introduce testosterone from an outside source, your brain registers that hormone levels are sufficient and stops sending the signals that tell the testes to produce sperm. In clinical studies, roughly 97% of men on regular testosterone injections had their sperm counts drop below 5 million per milliliter within six months (normal is above 15 million). About two-thirds of non-Asian men and nearly 90% of Asian men reached a complete absence of sperm.
This is one of the most important side effects for men who want to have children. Sperm production typically recovers after stopping TRT, but recovery can take months to over a year, and in some cases may not fully return. If preserving fertility is a priority, your prescriber can discuss alternatives that stimulate your body’s own testosterone production without shutting down sperm output.
Skin Changes and Acne
Testosterone drives oil production in the skin. Before starting testosterone therapy, roughly 6% of patients in one large cohort had acne. After starting, that number jumped to 31%. A smaller study tracking patients over just four months found facial acne rates climbing from 35% to 82%, and trunk acne from 15% to 88%. About 28% of people on testosterone report a history of moderate to severe acne at some point during treatment.
The acne tends to appear on the face, back, and chest. It’s caused by the same mechanism as teenage acne: androgens enlarge the oil glands and increase sebum output, which clogs pores. Standard acne treatments (topical retinoids, benzoyl peroxide, or in severe cases oral medications) work for testosterone-related breakouts, and the problem often improves as your body adjusts over the first year.
Breast Tissue Growth
Your body naturally converts a portion of testosterone into estrogen through an enzyme called aromatase. When you add more testosterone, more gets converted, and higher estrogen levels can stimulate breast gland tissue to grow. This is called gynecomastia, and it can range from mild puffiness around the nipples to noticeable breast development. It’s one of the more distressing side effects for men, but it’s usually caught early during monitoring.
The degree of conversion varies by individual. Men with more body fat tend to convert more testosterone to estrogen, since fat tissue contains higher levels of the conversion enzyme. If estrogen-related side effects develop, your prescriber may adjust your dose, change your delivery method, or in some cases add a medication that blocks the conversion process.
Prostate Effects and PSA Changes
TRT does not appear to cause prostate cancer based on current evidence. However, it can increase PSA levels, the blood marker used to screen for prostate problems, which can trigger biopsies and lead to earlier detection of cancers that might already be present. In a controlled trial, the average PSA increase after 12 months of testosterone was about 0.47 ng/mL. Five percent of men saw an increase of 1.7 ng/mL or more, and about 2% reached an absolute PSA above 4.0 ng/mL, the traditional referral threshold.
The Endocrine Society recommends a urological referral if your PSA rises by more than 1.4 ng/mL during the first year or hits an absolute value above 4.0 ng/mL (or above 3.0 if you’re at higher baseline risk for prostate cancer). PSA should be checked before starting therapy, at 3 to 12 months, and then according to standard screening guidelines for your age and risk profile.
Sleep Apnea
TRT can worsen or even trigger obstructive sleep apnea. Research published in the American Journal of Respiratory and Critical Care Medicine demonstrated that testosterone increases the collapsibility of the upper airway during sleep. Essentially, the muscles that keep your airway open become more prone to relaxing and closing when testosterone levels rise. This effect reversed after stopping hormone treatment.
If you already snore heavily, wake up gasping, or feel exhausted despite a full night’s sleep, mention it before starting TRT. Untreated sleep apnea is actually listed as a contraindication to starting therapy in some guidelines. If sleep quality worsens after starting treatment, a sleep study can clarify whether the airway is the problem.
Cardiovascular Risk
For years, the biggest open question about TRT was whether it increased heart attack and stroke risk. The TRAVERSE trial, published in the New England Journal of Medicine, was the largest randomized safety trial to address this directly. Over 5,000 men aged 45 to 80 who already had cardiovascular disease or elevated risk were followed for years. Major cardiac events occurred in 7.0% of the testosterone group and 7.3% of the placebo group, a statistically insignificant difference.
This was reassuring but not a clean bill of health. The study population was carefully selected and monitored, which may not reflect how TRT is prescribed in everyday practice. The elevated red blood cell risk described above is itself a cardiovascular concern. The current consensus is that TRT at standard doses does not appear to independently increase major cardiac events in monitored patients, but it’s not risk-free either.
Mood and Aggression
The “roid rage” stereotype doesn’t hold up at clinical TRT doses. Studies giving normal men supraphysiological doses, up to 600 mg of testosterone per week (several times higher than a typical TRT dose of 100 to 200 mg weekly), found no measurable increase in aggression or anger on self-reported scales. Most men on TRT report improved mood, energy, and motivation rather than irritability.
That said, mood fluctuations can occur, particularly with injection-based delivery where testosterone levels peak and then drop before the next dose. Some men notice irritability or low mood in the days before their next injection. Switching to more frequent, smaller injections or using a daily topical gel can smooth out these swings.
Risks of Topical Testosterone Transfer
If you use a gel or topical solution, there’s a real risk of accidentally exposing the people around you. Women and children who touch skin where testosterone gel has been applied can absorb enough to cause symptoms. In children, this can trigger early puberty, genital enlargement, and behavioral changes. In women, it can cause acne and unwanted hair growth.
After applying gel, let it dry completely, cover the area with clothing, and wash your hands thoroughly. If skin-to-skin contact happens before the area is washed, the other person should scrub the contact area with soap and water immediately. This risk is specific to topical formulations and doesn’t apply to injections or implanted pellets.
What Monitoring Looks Like
The Endocrine Society recommends a structured monitoring schedule during the first year. Blood work at 3 to 6 months checks your testosterone level (to confirm the dose is putting you in the right range) and your hematocrit. At 12 months, both are rechecked, along with PSA for men in the appropriate screening age group. After the first year, these checks move to an annual schedule. Your provider should also ask about symptom improvement, side effects, and treatment adherence at each visit.
Most side effects of TRT are manageable when caught early through routine blood work. The men who run into serious problems are typically those using testosterone without medical supervision, at doses far above what’s clinically appropriate, or without any lab monitoring at all.