Switching from Testosterone Cypionate to Testosterone Undecanoate
Yes, you can switch your patient from weekly testosterone cypionate to testosterone undecanoate, but the dosing regimen and administration requirements are completely different and require careful attention to the specific protocol.
Critical Differences Between Formulations
Testosterone Cypionate (Current Regimen)
- Your patient is likely receiving 50-100 mg weekly via intramuscular injection, which can be self-administered in the thigh or given by another person in the gluteal region 1
- This provides relatively stable testosterone levels with weekly dosing 1, 2
Testosterone Undecanoate (Replacement Option)
- Must be administered as 750 mg initially, followed by 750 mg at 4 weeks, then 750 mg every 10 weeks thereafter 1
- Must be administered as gluteal intramuscular injection ONLY - this cannot be self-administered in the thigh like cypionate 1
- The volume is 3 mL per injection, which is the maximum safe volume for gluteal IM injection 1
- Provides fewer yearly injections (approximately 4-5 per year after loading) with less fluctuation in testosterone levels compared to biweekly cypionate dosing 1
Conversion Protocol
Initial Dosing Schedule
- First injection: 750 mg (3 mL) gluteal IM 1
- Second injection: 750 mg at 4 weeks 1
- Maintenance: 750 mg every 10 weeks 1
Monitoring Requirements
- Measure testosterone levels 2-3 months after initiating undecanoate, targeting mid-normal values of 450-600 ng/dL 1, 2
- For undecanoate, check levels before the next scheduled injection (at trough) 1
- Once stable, monitor every 6-12 months 1
- Continue monitoring hematocrit/hemoglobin and PSA as with any testosterone therapy 2
Important Clinical Considerations
Advantages of Testosterone Undecanoate
- Long-acting formulation requiring only 4-5 injections per year after loading 3, 4
- Provides stable testosterone levels for 12 weeks 5
- Excellent safety profile with no serious side effects noted in long-term studies up to 8 years 3
- May improve patient compliance due to convenient dosing schedule 3, 4
Critical Limitations and Caveats
- Cannot be self-administered - requires healthcare provider or trained person to give gluteal injections 1
- Requires 1.5 inch needle for proper gluteal administration 1
- More expensive than testosterone cypionate (though fewer injections may offset some cost) 1
- The 3 mL volume must be administered in the gluteal site only; this volume is too large for other sites 1
Common Pitfalls to Avoid
- Do not attempt to use the same weekly dosing schedule - undecanoate has completely different pharmacokinetics requiring the specific 750 mg loading and maintenance protocol 1, 6
- Do not allow thigh self-administration - the 3 mL volume and formulation characteristics require gluteal administration only 1
- Do not skip the 4-week loading dose - the initial dose followed by a second dose at 4 weeks is essential to achieve steady-state levels before transitioning to 10-week intervals 1, 6
- Do not assume interchangeability with cypionate dosing - these are fundamentally different formulations with distinct administration requirements 5, 7
Practical Implementation
If your patient requires self-administration capability or prefers more frequent but smaller injections, testosterone cypionate (or enanthate) remains the better choice. However, if the patient has access to healthcare administration every 10 weeks and values fewer injections, testosterone undecanoate is an excellent alternative with proven long-term safety and efficacy 3, 4.