Naprogesic Tab Specially designed to treat the cause of period pain by inhibiting the production of the pain producing molecules, prostaglandins. Works best when taken the day before your period is due, or at the first sign of period pain or menstrual bleeding (whichever comes first) as this allows it to stop the production of prostaglandins before they can cause pain. Effective temporary relief from period pain symptoms for up to 8 hours.