What is the equivalent intramuscular (IM) dose of testosterone (cypionate or enanthate) for a patient taking 50 mg of testosterone topical?

From the Guidelines

The equivalent intramuscular (IM) dose of testosterone cypionate or enanthate for a patient taking 50 mg of testosterone topical would be approximately 75-100 mg administered every 1-2 weeks. When transitioning from topical to injectable testosterone, it's essential to consider the differences in bioavailability and pharmacokinetics between these formulations. Topical testosterone has variable absorption and first-pass metabolism, while IM testosterone provides more consistent blood levels but with peaks and troughs. The starting IM dose should be on the lower end of the range (75 mg weekly or 150 mg every two weeks) with subsequent dose adjustments based on serum testosterone levels measured midway between injections 1. Patients should be monitored for clinical response and potential side effects such as erythrocytosis, acne, or mood changes. Testosterone levels should be checked 4-6 weeks after initiating therapy to ensure appropriate dosing. The goal is to maintain serum testosterone levels within the mid-normal range while achieving symptom improvement. Some key points to consider when making this transition include:

• The potential benefits and harms of testosterone treatment, including the impact on sexual function, energy, vitality, physical function, and cognition 1
• The importance of discussing treatment options with patients and considering their preferences and values
• The need for regular monitoring and follow-up to ensure safe and effective treatment
• The consideration of intramuscular formulations as a cost-effective option with similar clinical effectiveness and harms compared to transdermal formulations 1

From the Research

Equivalent IM Dose of Testosterone

To determine the equivalent intramuscular (IM) dose of testosterone for a patient taking 50 mg of testosterone topical, we need to consider the pharmacokinetics and bioavailability of different testosterone formulations.

• The study by 2 compared the pharmacokinetics of subcutaneous testosterone enanthate with intramuscular testosterone enanthate. The results showed that 50 mg of subcutaneous testosterone enanthate achieved normal average concentrations of serum testosterone within a 168-hour dosing interval.
• Another study by 3 evaluated the efficacy of different dosage regimens of testosterone enanthate in treating male hypogonadism. The study found that 100 mg of testosterone enanthate once a week, 200 mg every 2 weeks, 300 mg every 3 weeks, and 400 mg every 4 weeks all produced serum testosterone concentrations that fluctuated largely within the normal range.
• However, there is limited direct evidence comparing the bioavailability and efficacy of topical testosterone with IM testosterone. The study by 4 compared the efficacy of topical and intramuscular testosterone in stimulating penile growth, but it did not provide a direct comparison of the equivalent doses.

Estimated Equivalent IM Dose

Based on the available evidence, it is difficult to estimate the exact equivalent IM dose of testosterone for a patient taking 50 mg of testosterone topical. However, considering the study by 2, which showed that 50 mg of subcutaneous testosterone enanthate achieved normal average concentrations of serum testosterone, it is possible that a similar dose of IM testosterone enanthate (e.g., 50-100 mg) may be equivalent to 50 mg of topical testosterone.

• The following factors should be considered when estimating the equivalent IM dose:
  • Bioavailability: Topical testosterone has a lower bioavailability compared to IM testosterone.
  • Dosing frequency: IM testosterone is typically administered every 1-4 weeks, while topical testosterone is applied daily.
  • Individual variability: Patients may respond differently to different testosterone formulations and doses.

In summary, while there is limited direct evidence to determine the equivalent IM dose of testosterone for a patient taking 50 mg of testosterone topical, a dose of 50-100 mg of IM testosterone enanthate may be estimated based on the available evidence 2, 3. However, this estimate should be used with caution, and individual patient responses should be monitored closely.