There are fascinating similarities and differences between aspirin and acetaminophen. Sometimes referred to by the alternate name, paracetamol, it is, like aspirin, an analgesic (pain reliever). Also like aspirin it has been used to treat headaches since the late 19th century. Yet there are interesting differences as well.

Aspirin works by interfering with the action of a type of natural hormone called prostaglandins. These biochemicals come into play during inflammation, such as occurs during injury. Hit your thumb with a hammer and the swelling starts. Prostaglandins stimulate signals through nerve cells that we perceive as pain, and stimulate the swelling. That swelling is the body's reaction to increase blood supply and surround damaged tissue.

But acetaminophen is not an NSAID (Non-Steroidal Anti-Inflammatory Drug) like aspirin or ibuprofen. Consequently, it does not reduce pain from conditions caused by inflammation, as aspirin will. Yet, unlike ibuprofen, it's safe to use during all stages of pregnancy. Ibuprofen is particularly to be avoided in the third trimester.

Acetaminophen has another advantage over aspirin in some cases. In certain diseases, such as Reye's syndrome, acetaminophen can be taken safely. Aspirin is unsafe for those afflicted. It also has many fewer side effects for a wide variety of people.

At proper dosages, acetaminophen doesn't irritate the stomach lining as aspirin can. It also does not carry the same risk of kidney damage as its pharmacological cousin. Aspirin thins the blood, which accounts in part for its use as a heart attack preventative. But clotting can be useful if one is subject to nose bleeds or gets cut. Acetaminophen does not interfere with clotting.

The reasons lie in the similarities and differences between the two closely related compounds. Aspirin interferes with prostaglandin production by inhibiting an enzyme called COX. Though acetaminophens don't reduce those inflammation hormones, it does, paradoxically, inhibit the COX enzymes used to produce them.

On the other hand, there is some risk associated with acetaminophen when taken at improperly high dosages. Daily use can lead to so-called rebound headaches, and at high enough dosages can cause liver damage and even death. Such risks are low, but are increased by, for example, taking it in conjunction with alcohol.

Some more common side effects to watch out for include bloody or black, tarry stools, skin rash and sore throat (not present before beginning treatment). In rare cases, lip ulcers or white spots in the mouth have been reported.

It is generally safe to take acetaminophen in conjunction with other over-the-counter pain control medicines. In fact, Excedrin contains aspirin, acetaminophen and caffeine. Any persistent fever or headache should be diagnosed by a physician.