Introduction Androgen deprivation therapy (ADT) is a mainstay of treatment against advanced prostate cancer (PC). As a treatment goal, suppression of plasma testosterone levels to <50 ng/dl has been established over decades. Evidence is growing though that suppression to even lower levels may add further clinical benefit. Therefore, we undertook a pooled retrospective analysis on the efficacy of 1-, 3-, and 6-month sustained-release (SR) formulations of the gonadotropin-releasing hormone (GnRH) agonist triptorelin to suppress serum testosterone concentrations beyond current standards. Methods Data of 920 male patients with PC enrolled in 9 prospective studies using testosterone serum concentrations as primary endpoint were pooled. Patients aged 42–96 years had to be eligible for ADT and to be either naïve to hormonal treatment or have undergone appropriate washout prior to enrolment. Patients were treated with triptorelin SR formulations for 2–12 months. Primary endpoints of this analysis were serum testosterone concentrations under treatment and success rates overall and per formulation, based on a testosterone target threshold of 20 ng/dl. Results After 1, 3, 6, 9, and 12 months of treatment, 79%, 92%, 93%, 90%, and 91% of patients reached testosterone levels <20 ng/dl, respectively. For the 1-, 3-, and 6-month formulations success rates ranged from 80–92%, from 83–93%, and from 65–97% with median (interquartile range) serum testosterone values of 2.9 (2.9–6.5), 5.0 (2.9–8.7), and 8.7 (5.8–14.1) ng/dl at study end, respectively. Conclusion In the large majority of patients, triptorelin SR formulations suppressed serum testosterone concentrations to even <20 ng/dl. Testosterone should be routinely monitored in PC patients on ADT although further studies on the clinical benefit of very low testosterone levels and the target concentrations are still warranted.