A 38-year-old woman presents to the GP with a 6-month history of chronic lower abdominal pain with no clear cyclical pattern. She has normal periods. Pelvic examination, pelvic ultrasound, and STI screening are all normal. She has no urinary or bowel symptoms. She describes the pain as constant and dull, rated 5/10. Which is the single most appropriate pharmacological agent to consider for chronic pelvic pain?