Testosterone replacement therapy (TRT) is prescribed to adult men with clinically confirmed testosterone deficiency — defined by symptoms (fatigue, low libido, erectile dysfunction, reduced muscle mass) plus persistently low serum testosterone measured on two separate morning samples. The 2018 American Urological Association (AUA) guideline on testosterone deficiency (Mulhall et al., Journal of Urology, 2018) frames the evidence base for replacement and emphasizes individualized route selection based on patient preference, cost, and monitoring tolerance.

The three major delivery routes — injectable, oral, and transdermal — differ meaningfully in pharmacokinetics, bioavailability, and practical considerations. This article compares them.

Medical Disclaimer

The information in this article is for general education only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a licensed healthcare provider before starting, stopping, or changing any medication. ZYNDIO connects adults with licensed providers via telehealth; the providers — not ZYNDIO — make all clinical decisions. Compounded medications dispensed through ZYNDIO partners are not FDA-approved drug products. They are prepared by state-licensed compounding pharmacies pursuant to a valid prescription. Individual results vary. Side effects, drug interactions, and contraindications exist for every therapy discussed here.

Injectable Testosterone: Cypionate, Enanthate, and Undecanoate

Injectable testosterone remains the most widely prescribed route in the United States. The two most common esters — testosterone cypionate and testosterone enanthate — are pharmacologically equivalent. Both have a half-life of approximately 8 days and are typically dosed intramuscularly at 50-100 mg weekly or 100-200 mg every two weeks.

Bioavailability and Peak-Trough Variability

Intramuscular injection bypasses first-pass hepatic metabolism, yielding high bioavailability (near 100%). After a single 200 mg injection of testosterone cypionate, serum testosterone peaks within 24-48 hours, then declines steadily. The AUC (area under the curve) for cypionate and enanthate is dose-proportional.

Peak-trough variability is common with biweekly dosing. Some patients report energy or mood fluctuations in the days before the next injection. Weekly or twice-weekly protocols reduce this variability by maintaining more stable serum levels.

Testosterone Undecanoate (Aveed)

Testosterone undecanoate is a long-acting injectable formulation. The FDA approved Aveed in 2014 for testosterone deficiency. It is administered as a 750 mg intramuscular injection at weeks 0 and 4, then every 10 weeks thereafter. The long half-life (approximately 33 days) results in sustained serum testosterone with minimal peak-trough swings.

Aveed carries an FDA-mandated Risk Evaluation and Mitigation Strategy (REMS) due to reports of pulmonary oil microembolism and anaphylaxis. It must be administered in a healthcare setting with 30 minutes of post-injection observation. This limits its use in telehealth and self-administration contexts.

Oral Testosterone Undecanoate (Jatenzo)

Oral testosterone has historically been limited by poor bioavailability and hepatotoxicity risk. Older oral formulations (17-alpha-alkylated androgens like methyltestosterone) are associated with cholestatic liver injury and are rarely used.

The FDA approved Jatenzo (oral testosterone undecanoate) in 2019. It is absorbed via the intestinal lymphatic system, bypassing first-pass hepatic metabolism to a degree. Jatenzo is dosed twice daily with food. Starting doses range from 237 mg to 396 mg per dose, adjusted based on serum testosterone measured 4-6 hours after the morning dose.

Hepatic Considerations

While Jatenzo avoids the severe hepatotoxicity of older oral androgens, liver enzyme monitoring is still recommended. The prescribing information advises checking alanine aminotransferase (ALT) and aspartate aminotransferase (AST) at baseline, at 3 months, and annually thereafter. Patients with existing liver disease are generally not candidates.

Oral testosterone undecanoate is contraindicated in patients with a history of myocardial infarction or stroke within the past 6 months, uncontrolled hypertension, or severe heart failure. The twice-daily dosing requirement and cost (Jatenzo is branded and expensive) limit uptake.

Transdermal Testosterone: Gels and Patches

Transdermal testosterone delivers the hormone through the skin, maintaining relatively stable serum levels throughout the day. Approved transdermal gels include AndroGel, Testim, Fortesta, and Vogelxo. Transdermal patches (Androderm) are less commonly prescribed due to skin irritation.

Application and Absorption

Gels are typically applied once daily to clean, dry skin on the shoulders, upper arms, or abdomen. Absorption occurs over several hours. Serum testosterone reaches steady state within 3-7 days of daily use.

The bioavailability of transdermal testosterone is lower than injectable — approximately 10-15% of the applied dose is absorbed. This is accounted for in the dosing: a typical starting dose is 40-50 mg of testosterone per day (which corresponds to 4-5 grams of gel, depending on the formulation).

Transfer Risk to Children and Partners

The FDA issued a black box warning for testosterone gels in 2009 due to reports of virilization in children exposed through skin contact with treated adults. Transfer risk is highest in the first few hours after application. Patients are instructed to:

  • Apply gel to areas covered by clothing
  • Wash hands thoroughly after application
  • Avoid skin-to-skin contact with children or pregnant/breastfeeding partners until the application site is washed (at least 6 hours after application, or after showering)

This risk is not theoretical. Case reports document early puberty and genital enlargement in children exposed to testosterone gels (PMID: 19509386, Clinical Pediatrics, 2009).

Patches have lower transfer risk because the adhesive system limits contact, but they cause local skin reactions (erythema, pruritus) in 30-60% of users.

Lab Monitoring Cadence: AUA Protocol

The 2018 AUA guideline recommends a structured monitoring schedule for all TRT patients, regardless of route:

  • 3 months after initiation: Total testosterone, hematocrit, and prostate-specific antigen (PSA) in men over 40
  • 6 months: Repeat total testosterone and hematocrit
  • Annually thereafter: Total testosterone, hematocrit, PSA (in men over 40), and liver function tests (if using oral testosterone)

Free testosterone and estradiol levels are not routinely required but may be measured in patients with symptoms despite normal total testosterone, or in those using aromatase inhibitors.

Polycythemia Risk

Elevated hematocrit (>54%) is the most common adverse effect of TRT and occurs in 10-20% of patients. It is more frequent with injectable testosterone than transdermal, likely due to higher peak serum levels. Polycythemia increases the risk of thrombotic events. Patients with hematocrit above 54% are typically advised to dose-reduce, switch routes, or undergo therapeutic phlebotomy.

Estradiol and SERM Use

Testosterone is aromatized to estradiol by the enzyme aromatase. Some patients on TRT develop estradiol-mediated side effects — gynecomastia, nipple sensitivity, or water retention. Selective estrogen receptor modulators (SERMs) like tamoxifen or raloxifene are sometimes prescribed off-label to manage these symptoms. Aromatase inhibitors (anastrozole) are less commonly used due to concerns about bone density and lipid profiles.

Estradiol levels are not routinely monitored in asymptomatic patients. The AUA guideline does not recommend universal estradiol testing.

Candidate Selection by Route

Injectable Testosterone

Appropriate for: Patients who prefer less-frequent dosing, tolerate injections, and are comfortable with self-administration (or have access to clinic-based administration). Weekly or twice-weekly protocols are ideal for patients sensitive to peak-trough variability.

Not appropriate for: Patients with needle phobia or bleeding disorders, or those who cannot tolerate intramuscular injections.

Oral Testosterone Undecanoate

Appropriate for: Patients who cannot use injections or transdermal due to personal preference or contraindications, and who are willing to dose twice daily with meals. Requires baseline and serial liver enzyme monitoring.

Not appropriate for: Patients with liver disease, recent cardiovascular events, or those who cannot adhere to twice-daily dosing.

Transdermal Testosterone

Appropriate for: Patients who prefer daily dosing, avoid injections, and can adhere to application precautions (especially those with young children or pregnant partners in the household).

Not appropriate for: Patients with significant skin conditions (eczema, psoriasis) at application sites, those with young children at home who cannot reliably avoid skin contact, or those who swim or shower frequently within hours of application.

Compounded Testosterone and Telehealth Context

ZYNDIO connects patients with licensed providers who may prescribe compounded testosterone preparations. Compounded testosterone products are not FDA-approved drug products. They are prepared by state-licensed compounding pharmacies under a valid prescription. The bioavailability, dosing protocols, and monitoring requirements for compounded testosterone are the same as for branded formulations of the same ester and route.

Telehealth consultations for TRT typically include:

  • Review of symptoms and prior lab work
  • Confirmation of two morning serum testosterone levels below the lower limit of normal (typically <300 ng/dL, though reference ranges vary)
  • Assessment of contraindications: active prostate cancer, breast cancer, untreated severe obstructive sleep apnea, uncontrolled heart failure, or hematocrit >50% at baseline
  • Discussion of route preference and lifestyle factors

Providers do not prescribe TRT without documented lab evidence of deficiency.

When to Talk to Your Provider

You should contact your provider if you experience:

  • Chest pain, shortness of breath, or leg swelling (possible clot)
  • Urinary symptoms that worsen (hesitancy, weak stream, nocturia)
  • Mood changes, aggression, or irritability
  • Nipple tenderness or breast enlargement
  • Persistent skin reactions (for transdermal users)

Any dosing adjustment or route change requires provider approval. Do not alter your dose or frequency without consulting your provider.

FAQ

Can I switch from injectable to transdermal testosterone if I don't like injections?

Yes, switching routes is common and straightforward. Your provider will adjust the dose based on the new route's bioavailability and order follow-up labs at 3 months to confirm therapeutic serum levels. Transdermal testosterone reaches steady state within a week, so you can typically discontinue injections and start gel the same day.

Is oral testosterone safer than injectable?

No route is universally "safer." Oral testosterone undecanoate (Jatenzo) avoids injection-site reactions and transfer risk but requires liver enzyme monitoring and twice-daily dosing. Injectable testosterone has higher bioavailability and is dosed less frequently but carries a higher risk of polycythemia. The choice depends on your medical history and preferences.

How long does it take to feel the effects of TRT?

Libido and energy improvements are typically reported within 3-6 weeks. Muscle mass and strength gains become apparent at 3-6 months. Bone density improvements take 12-24 months. Effects vary by individual and depend on maintaining therapeutic serum testosterone levels.

Do I need to check my estradiol level on TRT?

Not routinely. The AUA guideline does not recommend universal estradiol monitoring. Your provider may order an estradiol level if you develop symptoms of elevated estrogen (gynecomastia, water retention, mood changes) or if you are using a SERM or aromatase inhibitor.

Can I use testosterone gel if I have young children?

You can, but transfer risk must be managed carefully. Apply gel to areas covered by clothing, wash your hands immediately after application, and avoid skin contact with children until you have showered (at least 6 hours later). If you cannot reliably follow these precautions, injectable or oral testosterone may be safer options.

Last reviewed: 2026-04-25 by ZYNDIO Clinical Editorial Team (PharmD-led)