Testosterone Enanthate Side Effects: Incidence Rates Across Clinical Trials

Does the Enanthate Label Give Incidence Rates?

No. The adverse reactions section of the testosterone enanthate injection label is a plain list: gynecomastia, excessive frequency and duration of penile erections, oligospermia at high dosages, hirsutism, male pattern baldness, acne, myocardial infarction, stroke, fluid retention, nausea, cholestatic jaundice, liver function test changes, polycythemia, libido changes, headache, anxiety, depression, venous thromboembolism, and injection site pain and inflammation [1]. No percentages appear in that section. The warnings follow the same pattern: venous thromboembolic events are described as postmarketing reports, and gynecomastia as something that 'frequently develops and occasionally persists' [1].

Hematocrit and Erythrocytosis

The enanthate label says androgens 'have been reported to stimulate the production of red blood cells' and lists polycythemia among hematologic adverse reactions [1]. The size of that effect has been measured in pooled randomized data.

A Bayesian network meta-analysis of 29 placebo-controlled randomized trials covering 3,393 men found every testosterone formulation raised mean hematocrit significantly. Intramuscular testosterone enanthate or cypionate raised it by 4.0 percentage points (95% CI 2.9 to 5.1), gel by 3.0, patch by 1.4, oral testosterone undecanoate by 4.3, and intramuscular testosterone undecanoate by 1.6 [2]. Intramuscular enanthate or cypionate rose significantly more than patch; the other head-to-head comparisons were not significantly different [2].

Averages hide the men who cross a threshold. In a retrospective review of 178 hypogonadal men followed for 3 years at a single center, erythrocytosis — hematocrit above 50% — occurred in 66.7% of men receiving injectable testosterone [3]. The injectable group was younger (mean 42.5 years versus about 54 in the other groups) and started with higher baseline hematocrit, and the study was not randomized [3]. Two reviews reach the same qualitative conclusion: short-acting injectable formulations carry the highest incidence of erythrocytosis [7], and men on testosterone replacement had a 315% greater risk of erythrocytosis (hematocrit above 0.52) than controls [8]. Neither review found a settled link between that erythrocytosis and thromboembolic events [7][8].

Blood Pressure and Cardiovascular Events

On February 28, 2025, FDA called for TRAVERSE results to be added to testosterone labels, removal of boxed-warning language about increased cardiovascular risk, and a blood pressure warning for products that lacked one. FDA said ambulatory blood pressure studies confirmed a blood pressure increase with all testosterone products, class-wide [9].

That label warns that testosterone can increase blood pressure and that blood pressure should be monitored periodically [1]. It also still carries the older cardiovascular paragraph: long-term clinical safety trials have not been conducted, epidemiologic studies and randomized trials 'have been inconclusive' for major adverse cardiovascular events, and 'Some studies, but not all, have reported an increased risk of MACE' [1].

TRAVERSE, the trial behind the 2025 changes, tested a daily 1.62% testosterone gel, not an injection. In 5,246 men aged 45 to 80 with hypogonadism and existing or high cardiovascular risk, the primary composite of cardiovascular death, nonfatal heart attack or nonfatal stroke occurred in 7.0% on testosterone versus 7.3% on placebo — hazard ratio 0.96 (95% CI 0.78 to 1.17), meeting noninferiority [5]. About 61% of participants stopped study gel or placebo early [5].

The Testosterone Trials, a separate coordinated set of seven placebo-controlled trials in 788 men with a mean age of 72, used gel as well. Its cardiovascular trial found that testosterone increased coronary artery noncalcified plaque volume measured by computed tomographic angiography, while the overall program did not show more cardiovascular or prostate adverse events than placebo [10].

Sperm Production

Exogenous testosterone inhibits pituitary luteinizing hormone and, at large doses, follicle stimulating hormone, which suppresses sperm production; the enanthate label lists oligospermia at high dosages [1].

The numbers come from male contraception research at 200 mg of enanthate weekly, well above the label's 50 to 400 mg every 2 to 4 weeks [1]. In the 1990 WHO multicenter study of 271 healthy fertile men, 157 became azoospermic, a cumulative rate of 65% at 6 months. After injections stopped, the estimated median time to reach a sperm concentration of at least 20 million/mL was 3.7 months (95% CI 3.6 to 3.9), and to return to each man's baseline concentration 6.7 months (95% CI 6.2 to 8.7) [4].

A separate 1996 WHO study enrolled 399 healthy fertile men receiving 200 mg of intramuscular testosterone enanthate weekly [11]. Suppression was inadequate in 8 men and escaped in 4; 9.4% discontinued for medical reasons, with no serious treatment-related side effects reported, and fertility was later demonstrated in 33 couples [11].

A pooled time-to-event analysis of 30 studies and 1,549 healthy men on androgen or androgen-progestin regimens put the median time for sperm concentration to recover to 20 million/mL at 3.4 months [6]. Recovery was faster with shorter-acting testosterone preparations, shorter treatment and higher baseline sperm counts [6]. These were eugonadal volunteers, not men with hypogonadism, and the Endocrine Society guideline recommends against starting testosterone therapy in men planning fertility in the near term [12].

Mood, Skin, Breast Tissue, Lipids and Liver

The enanthate label names acne, hirsutism, male pattern baldness, increased or decreased libido, headache, anxiety and depression, increased serum cholesterol, cholestatic jaundice, alterations in liver function tests, and rarely hepatocellular neoplasms and peliosis hepatis with prolonged high-dose use [1]. Some of these effects have been measured directly in the studies below.

Gynecomastia is the one the label characterizes at all: it 'frequently develops and occasionally persists' in men treated for hypogonadism [1]. The mechanism is estrogenic rather than a direct testosterone effect — testosterone is converted to estradiol by aromatase, and estrogens stimulate breast tissue while androgens inhibit it, so most gynecomastia reflects excess estrogen action relative to androgen action [13][14].

For mood, the controlled data come from two studies at high doses. In a randomized crossover trial of 56 men given testosterone cypionate rising to 600 mg per week for 6 weeks, 42 of the 50 men who reached that dose (84%) showed minimal psychiatric effect, 6 (12%) became mildly hypomanic and 2 (4%) markedly hypomanic [15]. In a study that combined a hormone-suppressing drug with 25 to 600 mg of enanthate weekly, mood, sexual function and PSA did not change significantly at any dose, while HDL cholesterol fell as dose rose [16]. These findings do not establish a mood-effect rate for usual enanthate replacement treatment. That cypionate trial and that enanthate study are the source of the mood numbers — not the enanthate label, which simply lists anxiety and depression [1].

Injection-Site Reactions

The enanthate label lists inflammation and pain at the injection site and, rarely, anaphylactoid reactions, with no frequency attached [1].

Why the Numbers Do Not Transfer Between Studies

Three things separate one trial's percentage from another's.

Dose. Standard replacement is 50 to 400 mg every 2 to 4 weeks [1]. The WHO contraception studies used 200 mg every week [4][11], and dose-response work has used up to 600 mg weekly [16].

Formulation. TRAVERSE and the Testosterone Trials used a daily gel [5][10]. The mood figures come from a 56-man trial of high-dose testosterone cypionate, not from a trial measuring mood-side-effect rates during usual enanthate replacement treatment [15]. The hematocrit meta-analysis pooled enanthate and cypionate injections and found they behaved differently from patches [2].

Population. The WHO contraception studies involved healthy fertile men rather than men receiving replacement therapy for hypogonadism [4][11]; the plaque data come from men with a mean age of 72 [10]; and the cardiovascular outcome data come from men with existing or high cardiovascular risk [5]. Erythrocytosis rates in a single-center review were confounded by age and baseline hematocrit [3].