Evaluating the Efficacy and Safety of 48-Week Low-Dose Dienogest Administration in Patients With Dysmenorrhea Caused by Endometriosis: Protocol for a Randomized, Open-Label, Parallel-Group Trial.

Background: The treatment of endometriosis includes analgesics, hormone therapy, and surgery. Even after surgical removal of endometriotic lesions, the risk of recurrence remains high once the normal menstrual cycle resumes. Therefore, long-term hormone therapy is essential to prevent recurrence. Among hormonal treatments, low-dose estrogen progestin preparations are not recommended for patients older than 40 years due to the increased risk of thrombotic side effects. In contrast, dienogest does not carry a thrombotic risk, making it a suitable option for older patients. Although dienogest requires long-term administration until menopause in patients with endometriosis, data on its long-term efficacy and potential adverse effects remain limited. In particular, comparative studies assessing the safety and effectiveness of long-term use of dienogest at different doses (1 mg/day vs 2 mg/day) have not been conducted, highlighting the need for further investigation.

Objective: The purpose of this study is to investigate the efficacy and the incidence of adverse events of dienogest 1 mg/day after 48 weeks in patients with dysmenorrhea due to endometriosis, compared with dienogest 2 mg/day.

Methods: This randomized, open-label, parallel-group, dose-comparison, multicenter trial follows the SPIRIT (Standard Protocol Items: Recommendations for Interventional Trials) 2013 guidelines and is conducted at 6 centers in Japan. Participants are randomly assigned in a 1:1 ratio to receive either dienogest 1 mg/day or 2 mg/day. The drug is administered for 48 weeks, and its therapeutic effects and side effects are evaluated. Hospital visits include the use of questionnaires, vital sign measurements, imaging studies (magnetic resonance imaging and ultrasound), blood tests, and bone density assessments. The primary endpoint is the change in the pain visual analog scale (VAS) score from baseline to 48 weeks. The VAS is a 10 cm horizontal scale where 0 cm represents no pain and 10 cm represents the maximum imaginable pain; participants indicate their pain level on the scale, and the change is analyzed over time. The target sample size is 88, determined with a noninferiority margin based on existing literature. The protocol was approved by the Nagoya City University Hospital Clinical Research Review Board. Findings will be presented at academic conferences and published in peer-reviewed journals.

Results: Currently, data collection is ongoing. The first participant was enrolled in August 2021. As of March 22, 2025, a total of 88 participants had been enrolled in this clinical trial.

Conclusions: This is the first trial to compare efficacy and safety between 1 mg/day and 2 mg/day of long-term dienogest use in patients with dysmenorrhea caused by endometriosis. Combining diagnostic imaging with patient questionnaires and blood tests allows the determination of efficacy against endometriosis itself.

Trial registration: Japan Registry of Clinical Trials jRCTs041210016; https://jrct.mhlw.go.jp/en-latest-detail/jRCTs041210016.

International registered report identifier (irrid): DERR1-10.2196/66246.