Vaginal Sildenafil for Primary Dysmenorrhea

Inclusion Criteria: * Female sex assigned at birth * Age 18 to 35 years * History of moderate-to-severe dysmenorrhea * Regular menstrual cycles of approximately 21 to 35 days * Willing and able to complete a single menstrual treatment visit * Able to comply with study procedures, including MRI procedures and vaginal self-administration of study product * Able to provide informed consent Exclusion Criteria: * Gross pelvic pathology identified by clinical history or by the prespecified review of the baseline research MRI. This criterion does not apply to participants enrolled in the prespecified exploratory cohort with known endometriosis or known uterine fibroids, who are analyzed separately. * Known hypersensitivity to sildenafil or any formulation component * Use of nitrates or nitric oxide donors * Use of any CYP3A4 inhibitor or inducer, including dietary sources such as grapefruit * Use of another phosphodiesterase type 5 inhibitor * Use of any alpha-adrenergic blocker or any antihypertensive medication * Uncontrolled hypertension, defined as systolic blood pressure at or above 140 mmHg or diastolic blood pressure at or above 90 mmHg * Near-hypotension, defined as systolic blood pressure at or below 95 mmHg or diastolic blood pressure at or below 65 mmHg * Cardiovascular disease * Clinically significant electrocardiographic abnormality, as judged by the investigator * Clinically significant laboratory abnormality, as judged by the investigator * Hepatic impairment of any severity, assessed by screening hepatic laboratory testing and clinical history * Severe renal impairment * Platelet count below 100,000 per microliter or hemoglobin below 10 g/dL on screening complete blood count * Bleeding disorder * Peptic ulcer disease * History of non-arteritic anterior ischemic optic neuropathy, risk factors for non-arteritic anterior ischemic optic neuropathy, or other retinal disorders * Sickle cell disease * Active pelvic infection * Presence of an intrauterine device, because of MRI artifact affecting interpretability * Other contraindication to MRI or factors that would impair MRI safety or interpretability, including certain metallic implants, metallic injury, or claustrophobia * Pregnant or breastfeeding * Any condition that, in the opinion of the investigator, would increase risk or interfere with study participation