Vaginal Sildenafil for Primary Dysmenorrhea

Inclusion Criteria: * Female sex assigned at birth * Age 18 to 35 years * History of moderate-to-severe primary dysmenorrhea * Regular menstrual cycles of approximately 21 to 35 days * Willing and able to complete 2 menstrual treatment visits in separate menstrual cycles * Able to provide informed consent * Able to comply with study procedures, including MRI procedures and vaginal self-administration of study product Exclusion Criteria: * Known hypersensitivity to sildenafil or any formulation component * Current use of nitrates or nitric oxide donors * Current use of potent CYP3A4 inhibitors or potent CYP3A4 inducers * Current use of another phosphodiesterase type 5 inhibitor * Uncontrolled hypertension, near-hypotension, or other clinically significant hemodynamic instability * Clinically significant cardiovascular disease * Clinically significant electrocardiographic abnormality, as judged by the investigator * Clinically significant laboratory abnormality, as judged by the investigator * Severe hepatic impairment or severe renal impairment * Active pelvic infection * Contraindication to MRI or factors that would impair MRI safety or interpretability, including certain metallic implants, metallic injury, claustrophobia, or intrauterine device-related artifact * Pregnant or breastfeeding * Any condition that, in the opinion of the investigator, would increase risk or interfere with study participation