How over-the-counter painkillers work

Over-the-counter painkillers fall into two groups. The first, nonsteroidal anti-inflammatory medications (NSAIDs), includes ibuprofen (often sold as Advil and Motrin), naproxen (sold as Aleve) and aspirin. All three drugs dull pain and fight inflammation.

Acetaminophen (often sold as Tylenol) is in a group of its own and targets only pain.

Scientists are still working out the details about how these drugs work.

NSAIDs are the better understood. In general, this group attacks two types of enzymes that produce prostaglandins, a broad class of chemicals that do a wide variety of jobs throughout our bodies. Among these, some play roles in sending pain signals to the brain. Others produce inflammation. Still others instigate fevers.

Taking doses of NSAIDs according to directions on the bottle generally targets just the pain-producing prostaglandins, says Janet Engle, a pharmacist at the University of Illinois at Chicago and a member of the Non-prescription Drugs Advisory Committee at the U.S. Food and Drug Administration. Doctors sometimes prescribe higher doses to also reduce the number of inflammation-inducing prostaglandins.

What sets one NSAID apart from another, for the most part, is how quickly their effects wear off — naproxen lasts the longest and aspirin lasts the shortest. Scientists think that each type of NSAID may also attack a different set of prostaglandin-producing enzymes, though that is less well understood.

Acetaminophen may or may not affect prostaglandins. Instead, this drug appears to affect the nervous system in other ways that have yet to be deciphered — allowing it to fight pain and fevers but not inflammation.

—Emily Sohn