Mefenamic acid is a type of medicine called a non-steroidal anti-inflammatory drug (NSAID).
It works by blocking the action of a substance in the body called cyclo-oxygenase (COX). Cyclo-oxygenase is involved in the production of various chemicals in the body, some of which are known as prostaglandins. Prostaglandins are produced by the body in response to injury and certain diseases and conditions, and cause pain, swelling and inflammation. Mefenamic acid blocks the production of these prostaglandins and is therefore effective at reducing inflammation and pain.
Mefenamic acid may also work by preventing the action of prostaglandins after they have already been formed.
Mefenamic acid helps with heavy and painful menstrual periods because period pain cramps are caused by the production of prostaglandins in the lining of the womb. Prostaglandin production is also known to be increased in women who suffer from heavy periods. By opposing the production and action of prostaglandins, mefenamic acid relieves period pains and also reduces blood loss in women who have heavy periods.