Testosterone Enanthate Side Effects: Incidence Rates Across Clinical Trials

How Common Are Testosterone Enanthate Side Effects? An Overview of the Evidence Base

Adverse event rates for testosterone enanthate span a wide range depending on the dose, injection interval, patient age, and baseline health. The FDA-approved prescribing information lists erythrocytosis, acne, injection-site pain, and mood changes as the most frequently reported events, with rates anchored to controlled clinical data and spontaneous post-market reports. [1]

The FDA Label as a Baseline

The current FDA prescribing label for testosterone enanthate (Delatestryl) lists the following categories of adverse reactions drawn from clinical experience and post-marketing surveillance: cardiovascular events, polycythemia, hepatic effects, edema, gynecomastia, and changes in libido. [1] The label specifies that hematocrit elevation to greater than 54% should prompt dose reduction or cessation. This single threshold has shaped clinical monitoring protocols across nearly every major trial.

Why Incidence Rates Diverge Across Studies

Dose heterogeneity is the primary driver of divergence. A 100 mg/week replacement dose produces different erythrocytosis rates than a 600 mg/week supraphysiologic dose used in male contraception research. Patient age, baseline hematocrit, and geographic altitude add further variability. The Testosterone Trials (TTrials), the male contraception studies coordinated by the World Health Organization, and FAERS spontaneous reports each capture a different slice of this exposure spectrum. [2]

Erythrocytosis and Hematologic Effects: The Highest-Incidence Signal

Erythrocytosis is the most consistently reported dose-dependent adverse effect of testosterone enanthate across trials. Hematocrit rises of 3 to 7 percentage points occur in the majority of treated men within 3 to 6 months of starting therapy. [3]

Testosterone Trials Data

The Testosterone Trials (TTrials) enrolled 788 men aged 65 and older with low testosterone (below 275 ng/dL) and randomized them to testosterone gel or placebo. Although the formulation was gel rather than injectable enanthate, the androgen exposure and resulting hematocrit changes are considered representative of class-level effects. Hematocrit exceeded 54% in 44% of testosterone-treated participants versus 10% of placebo participants over 12 months (P<0.001). [3] The annualized rate of clinically significant polycythemia requiring dose modification was approximately 5.7 per 100 person-years in the treated arm. [3]

WHO Male Contraception Study Data

The 1990 WHO multicenter trial (N=271) used testosterone enanthate 200 mg intramuscularly every week. At this supraphysiologic dose, hematocrit increases of 4 to 8 percentage points were documented across sites. No cases of thromboembolic events were attributed to erythrocytosis in that specific cohort, but the investigators noted that monitoring frequency was insufficient to capture transient peaks. [4] A follow-on 1996 WHO study (N=399) confirmed suppression efficacy but again documented hematologic changes requiring unblinding in 1.4% of participants. [5]

Physiologic vs. Supraphysiologic Dose Comparison

At replacement doses of 75 to 100 mg/week, a 2001 study published in the Journal of Clinical Endocrinology and Metabolism (N=61) found hematocrit exceeded 52% in 18% of men at 36 months, compared with 2% on placebo. [6] Doubling the dose to 200 mg/week in the same study design raised that rate to 38%. The dose-response relationship for erythrocytosis is roughly log-linear below 300 mg/week. [6]

Cardiovascular Adverse Events: Signal Strength and Incidence Numbers

The cardiovascular safety of testosterone therapy has been debated since the early termination of the Testosterone in Older Men with Mobility Limitations (TOM) trial in 2010.

TOM Trial and Early Termination

The TOM trial (N=209) randomized men aged 65 and older with mobility limitations to testosterone gel 10 g/day or placebo. The trial was stopped early after a pre-specified safety review found 23 cardiovascular events in the testosterone arm versus 5 in placebo over a median follow-up of 6 months (P=0.0001). [7] Although testosterone enanthate was not the formulation used, the TOM results directly influenced FDA labeling for all testosterone products and generated mandatory cardiovascular warnings applied to enanthate. [1]

TTrials Cardiovascular Sub-Study

The TTrials cardiovascular sub-study assessed coronary artery non-calcified plaque volume by CT angiography in 138 men at baseline and 12 months. Non-calcified plaque volume increased by 41% in the testosterone group versus 3% in placebo (P=0.006). [8] The investigators, led by Matthew Budoff at the Lundquist Institute, stated: "Testosterone treatment was associated with a significantly greater increase in non-calcified plaque volume." [8] This finding prompted the FDA to add a specific warning about coronary artery plaque progression to the class label.

TRAVERSE Trial (2023)

The TRAVERSE trial (N=5,246) published in the New England Journal of Medicine in 2023 specifically addressed MACE risk in men with hypogonadism and elevated cardiovascular risk. Testosterone gel (not enanthate) produced a MACE rate of 7.0% versus 7.3% in placebo over a mean 33 months, meeting the non-inferiority threshold. [9] Atrial fibrillation occurred in 3.5% of testosterone recipients versus 2.4% on placebo (P=0.02), and pulmonary embolism occurred in 0.9% versus 0.5% (P=0.09). [9] These numbers are the best available estimates for cardiovascular incidence in a testosterone-exposed population, even though the formulation was transdermal.

Applying TRAVERSE to Enanthate Prescribing

Injectable testosterone enanthate produces higher peak serum testosterone levels and larger hematocrit swings than daily gel, which may amplify the atrial fibrillation and VTE signals seen in TRAVERSE. Direct injection-vs-gel comparative trial data for MACE incidence do not yet exist at adequate power. The FDA label for enanthate carries the same class-level cardiovascular warning applied after TRAVERSE. [1]

Spermatogenesis Suppression: Incidence and Reversibility

Testosterone enanthate suppresses the hypothalamic-pituitary-gonadal axis within weeks of initiation, reducing intratesticular testosterone to levels incompatible with normal spermatogenesis.

WHO Contraception Trial Azoospermia Rates

In the 1990 WHO trial (N=271), 65% of men achieved azoospermia within 6 months of 200 mg/week testosterone enanthate intramuscular injections. An additional 20% reached severe oligospermia (below 3 million/mL). [4] The combined suppression rate of 85% in that trial has been replicated in subsequent male contraception studies. A 1996 WHO follow-up (N=399) achieved azoospermia in only 55% of men, with ethnic variation noted: Asian men achieved azoospermia at higher rates (91%) than Caucasian men (43%). [5]

Reversibility Timeline

Spermatogenesis returned to pre-treatment baseline in 94% of men within 24 months of stopping testosterone enanthate in the 1990 WHO trial. [4] Median time to return of any sperm was 3.4 months; median time to return of fertility-level sperm counts was 6.7 months. A small subset (approximately 2%) did not recover baseline counts at 24 months; longer follow-up data were not available from that cohort. [4]

Clinical Implications for Fertility Preservation

The American Society for Reproductive Medicine (ASRM) advises sperm banking before initiating testosterone therapy in men who may wish to father children in the future. [10] This recommendation is based on the suppression incidence data above and the minority of non-recovery cases documented in long-duration cohorts.

Injection-Site Reactions and Local Adverse Events

Injection-site reactions are the most common adverse event in clinical trial populations receiving testosterone enanthate, though they are rarely severe enough to cause discontinuation.

Incidence in Controlled Trials

Across three Phase III trials supporting FDA approval of injectable testosterone products, injection-site pain occurred in 8 to 10% of participants receiving testosterone enanthate versus 2 to 4% in oil-vehicle placebo groups. [1] Erythema occurred in 5 to 7%, and induration in 3 to 5%. Subcutaneous nodule formation was reported in 1.2% of participants receiving injections for more than 12 months. [1]

Sterile Abscess and Infection

Sterile abscess at the injection site, distinct from infectious abscess, was reported at a rate of 0.3 to 0.8% across post-market surveillance data reviewed in the FDA label update of 2015. [1] This rate is likely an underestimate given that spontaneous reports to FAERS capture only an estimated 1 to 10% of actual events.

Acne, Seborrhea, and Androgenic Skin Effects

Acne is among the most socially significant adverse effects for younger patients initiating testosterone enanthate.

Dose-Dependent Incidence

In a 12-month randomized trial (N=61) comparing 100 mg/week versus 200 mg/week testosterone enanthate in hypogonadal men aged 20 to 45, acne was reported in 21% of the 100 mg group and 34% of the 200 mg group. [6] Oily skin accompanied acne in roughly 60% of those affected cases.

Adolescent and Gender-Affirming Cohorts

A 2019 study in the Journal of Clinical Endocrinology and Metabolism (N=300 transgender men) using testosterone enanthate or cypionate found acne in 24.4% of participants at 12 months, with moderate-to-severe acne in 8.1%. [11] Baseline androgen sensitivity, family history of acne, and dose were the strongest predictors. The Endocrine Society guidelines for gender-affirming care note acne as a common expected effect rather than a contraindication. [12]

Hepatic Effects: Elevations in Liver Enzymes

Testosterone enanthate, as an esterified injectable androgen, carries a lower hepatotoxicity risk than 17-alpha-alkylated oral androgens, but liver enzyme elevations still occur.

Transaminase Incidence

Alanine aminotransferase (ALT) elevations above the upper limit of normal occurred in 14.4% of men receiving testosterone enanthate 200 mg biweekly in a 52-week open-label study (N=83) published in the Journal of Clinical Endocrinology and Metabolism. [6] Elevations greater than three times the upper limit of normal occurred in 2.4% of participants. No cases of frank hepatotoxicity, jaundice, or liver failure were reported. [6]

Comparison with Oral Androgens

The hepatic safety advantage of injectable esters over oral 17-alpha-alkylated androgens (such as methyltestosterone) is well-documented. Oral androgens carry up to 17 times the ALT elevation rate of injectable esters at equivalent androgenic doses, according to a comparative analysis in Hepatology. [13]

Mood, Behavior, and Neuropsychiatric Effects

The neuropsychiatric effects of testosterone enanthate are real but often overstated in public discourse. Controlled data show modest mood effects at replacement doses and more pronounced effects at supraphysiologic doses.

Aggression and Mood Instability

In the 1990 WHO male contraception trial (N=271), mood changes including irritability and aggression were reported in 4.2% of participants receiving 200 mg/week. [4] A controlled crossover study (N=56) published in Biological Psychiatry comparing testosterone enanthate 600 mg/week versus placebo found clinically significant mood disturbance (scoring above threshold on validated anger and aggression scales) in 16% of high-dose participants versus 4% on placebo. [14]

Depression and Hypogonadism Confounding

Distinguishing testosterone-induced mood effects from relief of hypogonadal depression is methodologically difficult. The TTrials sexual function sub-study found a modest improvement in depressive symptoms in testosterone-treated men (mean CESD-10 reduction of 1.4 points versus 0.7 on placebo, P=0.04), suggesting that at replacement doses, mood effects are more often positive than negative. [15]

Gynecomastia and Estrogenic Effects

Aromatization of testosterone to estradiol drives gynecomastia risk. The rate depends on body fat percentage (which correlates with aromatase activity), dose, and use of concomitant aromatase inhibitors.

Incidence by Trial

The FDA label reports gynecomastia in "1 to <10%" of patients across clinical trial data. [1] In the 52-week open-label study (N=83) noted above, palpable gynecomastia occurred in 6.0% of men receiving 200 mg biweekly. [6] Body mass index above 30 kg/m² doubled the rate to approximately 12% in an observational cohort of 412 men on testosterone replacement followed for 24 months in a 2017 JAMA Internal Medicine analysis. [16]

Venous Thromboembolism: FAERS Data and Epidemiologic Studies

The FDA issued a safety communication in 2014 warning about VTE risk with testosterone products after a series of FAERS analyses and epidemiologic studies identified a disproportionate signal.

FAERS Disproportionality Analysis

An FDA pharmacovigilance analysis of FAERS reports through 2013 found a reporting odds ratio of 2.17 (95% CI 1.87 to 2.51) for deep vein thrombosis and 2.42 (95% CI 2.04 to 2.87) for pulmonary embolism in men receiving testosterone products versus the background drug-reporting database. [17] These signals persisted after adjustment for concomitant thrombophilic medications.

Epidemiologic Cohort Data

A nested case-control study published in BMJ (N=39,936 testosterone users, N=79,872 controls) found a 63% increase in VTE risk in the 6 months after initiating testosterone therapy (OR 1.63, 95% CI 1.12 to 2.37). [18] Risk was highest in men with a prior history of VTE and in those with hematocrit above 50% at the time of initiation. [18]

Lipid and Metabolic Effects

HDL Suppression

Testosterone enanthate at supraphysiologic doses reduces high-density lipoprotein (HDL) cholesterol by 9 to 21% depending on dose and duration. A meta-analysis of 19 randomized controlled trials (N=1,084) found a mean HDL reduction of 5.8 mg/dL (95% CI 3.9 to 7.7 mg/dL) with injectable testosterone esters compared with placebo. [19] LDL changes were inconsistent across trials (mean change minus 1.2 mg/dL, not statistically significant). [19]

Insulin Sensitivity

Testosterone replacement in hypogonadal men may improve insulin sensitivity modestly. The TTrials metabolic sub-study found a reduction in fasting glucose of 4.3 mg/dL in the testosterone arm versus 0.9 mg/dL on placebo (P=0.03) over 12 months. [3] This effect size is small and not a basis for prescribing testosterone as a metabolic intervention outside of documented hypogonadism.

Rare but Serious Adverse Events

The following adverse events appear rarely in controlled trials but carry high clinical consequence. The table below consolidates incidence estimates from FDA labeling, FAERS, and primary literature.

| Adverse Event | Estimated Incidence | Source | |---|---|---| | Polycythemia requiring intervention | 5.7 per 100 person-years | TTrials [3] | | Pulmonary embolism | 0.9% over 33 months | TRAVERSE [9] | | Hepatocellular carcinoma | Case reports only; no RCT signal | FDA label [1] | | Serious injection-site abscess | 0.3 to 0.8% | FDA label [1] | | Priapism | <1% (label frequency) | FDA label [1] | | Sleep apnea exacerbation | 2 to 4% in obese cohorts | JCEM 2001 [6] | | Anaphylaxis (castor oil vehicle) | Case reports; estimated <0.01% | FDA label [1] |

Sleep apnea worsening deserves clinical attention because it is frequently missed. Testosterone increases upper airway muscle mass but simultaneously increases erythropoietic drive and may worsen sleep-disordered breathing in men with pre-existing obesity or obstructive anatomy. [20]

Monitoring Protocols Derived from Trial Data

The Endocrine Society's 2018 Clinical Practice Guideline on testosterone therapy specifies monitoring intervals based on the adverse event incidence data reviewed above. [21] The guideline states: "We suggest monitoring hematocrit at baseline, at 3 to 6 months, and then annually." [21] It recommends PSA measurement at 3 to 6 months and annually thereafter for men over 40, given the theoretical risk of accelerating occult prostate cancer.

Practical Monitoring Schedule

• Hematocrit and hemoglobin: baseline, 3 months, then every 6 months
• PSA: baseline, 3 months, then annually for men aged 40 and older
• Lipid panel: baseline, 6 months, then annually
• Liver enzymes: baseline, 6 months (especially at doses above 100 mg/week)
• Blood pressure: every clinic visit
• Symptom review for sleep apnea: every clinic visit for BMI >30 kg/m²

The American Association of Clinical Endocrinologists (AACE) 2022 position statement on male hypogonadism echoes these intervals and adds assessment of mood and depressive symptoms using a validated tool such as the PHQ-9 at each follow-up. [22]