Background
Quarterly intramuscular injections with long-acting testosterone undecanoate (TU) provide stable serum testosterone concentrations over time and are therefore preferred by many testosterone-deficient patients. However, the use of long-acting TU in elderly patients is limited due to lack of safety and feasibility studies.

Objective
To investigate long-acting TU pharmacokinetics and assess differences in treatment regimens and risk of adverse outcomes in younger versus elderly testosterone-deficient patients.

Materials and methods
Single-center longitudinal observational study. Patients who initiated long-acting TU treatment between 2005 and 2010 were included. Elderly patients were born before 1956 and younger patients between 1965 and 1985. TU dose was adjusted yearly through shortening or prolongation of time between injections. Treatment targets were as follows: (1) free testosterone between 0 and −1 SD from the age-adjusted mean, (2) no symptoms of testosterone deficiency, and (3) hematocrit within the normal range.

Results
The study population consisted of 63 elderly and 63 younger patients. Median follow-up time during testosterone replacement was 12.1 years. Increasing intervals between TU injections were performed 44% more often in the elderly compared to younger patients and time between TU injections were prolonged 4% more in the elderly patients. The hematocrit, as well as the hematocrit for a given serum testosterone (hematocrit: testosterone ratio), increased with treatment time but did not differ between age groups. During follow-up, 40% of patients—both elderly and younger—experienced polycythemia. Risk of polycythemia did not differ with age.

Discussion and conclusion
An increased number of adjustments of TU dose are necessary in elderly patients in order to reach treatment targets. TU treatment in elderly testosterone-deficient patients is not associated with an increased risk of polycythemia compared to younger patients if age-adjusted treatment targets are reached.