What is the half-life of testosterone enanthate?

Testosterone Enanthate Half-Life

The elimination half-life of testosterone enanthate is approximately 4-5 days, though clinical effects persist longer due to the depot formulation requiring dosing every 1-2 weeks to maintain therapeutic testosterone levels. 1, 2

Pharmacokinetic Properties

Terminal Elimination Half-Life

• Research studies measuring serum testosterone levels after intramuscular injection demonstrate that testosterone enanthate has a terminal elimination half-life of approximately 4-5 days in hypogonadal men 1, 2
• Despite this relatively short half-life, the depot formulation creates a sustained-release effect that maintains therapeutic testosterone levels for extended periods 1

Clinical Duration of Action

• Following a 200 mg intramuscular injection, eugonadal testosterone levels are maintained through day 11, with the testosterone:estradiol ratio remaining at or above baseline through day 21 2
• After 100 mg injections, eugonadal testosterone levels are maintained through day 7 2
• Serum testosterone concentrations fluctuate largely within the normal range between doses, with the average concentration highest with more frequent lower doses (100 mg weekly) and lowest with less frequent higher doses (400 mg every 4 weeks) 1

Recommended Dosing Based on Pharmacokinetics

Standard Replacement Regimens

• The most effective dosing regimens are 200 mg every 2 weeks or 300 mg every 3 weeks, based on suppression of serum LH to normal levels and practical administration frequency 1
• Alternative recommendation: 200 mg every 10-14 days for hypogonadal men to maintain consistent eugonadal hormone levels 2
• Weekly dosing of 100 mg provides the most stable testosterone levels but requires more frequent administration 1

Clinical Monitoring Implications

• Testosterone levels should be measured midway between injections (targeting 500-600 ng/dL) when monitoring patients on testosterone enanthate 3
• This timing accounts for the fluctuating serum levels that occur with injectable testosterone esters, which spend time in both supratherapeutic and subtherapeutic ranges between injections 3, 4

Comparison to Other Testosterone Formulations

Longer-Acting Injectable Alternatives

• Testosterone undecanoate has a significantly longer terminal elimination half-life of 18-23 days with a mean residence time of 21-23 days, allowing for dosing intervals of 6-10 weeks 5
• This represents a substantial improvement over testosterone enanthate's requirement for bi-weekly injections 5

Transdermal Formulations

• Transdermal testosterone preparations avoid the peaks and valleys associated with testosterone enanthate injections, providing more stable day-to-day testosterone levels 3, 4
• However, injectable testosterone enanthate remains relatively inexpensive and offers flexible dosing without risk of transference to others 3

Important Clinical Considerations

Withdrawal Period for Antiandrogens

• The half-life of antiandrogens varies considerably: bicalutamide has a half-life of approximately 1 week, while flutamide's major active form has a plasma half-life of only 5-6 hours 3
• Antiandrogen discontinuation responses typically require 4-8 weeks to assess, depending on the specific agent's half-life 3

Practical Pitfalls

• The 400 mg every 4 weeks regimen, while seemingly convenient, fails to adequately suppress LH concentrations to normal and produces the lowest average testosterone concentrations between doses 1
• Injectable testosterone produces supraphysiological testosterone and estradiol levels for several days after each injection, which may contribute to excessive erythropoiesis stimulation 4