ST. LOUIS — Dr. Philip Majerus discovered a few decades ago that aspirin can help prevent heart attacks and stroke. Since then, Majerus, a professor at Washington University, has maintained a deep respect for this little white pill, which has been around since the 1800s.

"Aspirin is the most effective drug that we have," Majerus says. "If we eliminated all but three drugs, aspirin would be one" we should keep, he says.

Much of America agrees with him. Aspirin is in virtually everyone's medicine cabinet.

Doctors still like it because, while there's a pill for every ailment aspirin addresses, aspirin is the only pill that takes care of so many conditions. Consumers still like it because it's the least expensive and most versatile drug on the market.

"To the consumer, it is almost a miracle drug," says Nimita Thekkepat, assistant professor of pharmacy at St. Louis College of Pharmacy. "It helps inflammation, fever, and it can save your life (from heart attack)."

Aspirin works by blocking the production of prostaglandins, the on-off switch in cells that regulate pain and inflammation, among other things. That's why aspirin stops mild inflammation and pain. But that's the blessing and the curse.

Prostaglandins are unique to each family of cells. Some versions are good; some versions promote pain; some versions constrict blood vessels and help platelets clot.

But to aspirin, the only good prostaglandin is a dead prostaglandin.

So while it blocks prostaglandins that cause pain or cause clotting and narrowing of the blood vessels and inflammation, it also blocks prostaglandins that protect the stomach lining.

That's where stomach bleeding comes from when you take aspirin or other drugs in the aspirin family. They're known as nonsteroidal anti-inflammatory drugs, or NSAIDs. Drugs in this family include ibuprofen (Motrin, Advil), naproxen (Aleve) and ketoprofen (Orudis). Aspirin causes the most irritation.

"Aspirin is the most toxic (painkiller) to the stomach," says Dr. Richard Brasington, head of the Division of Rheumatology at Washington University School of Medicine.

Still, the versatility of aspirin allows a range of dosages for physical problems. For example, a very low dose — one pill of 81 milligrams compared with two pills of 325 milligrams for a full dose — will protect your cardiovascular system by preventing blood clots and relaxing constricted blood vessels, but it rarely upsets the stomach.

"Remember that the low-dose aspirin for (cardiovascular health) is a completely different drug than taking 20 aspirin a day for pain," Brasington says.

Majerus agreed: "The effect on platelets is long-lasting at such a low dose that it doesn't affect anything else."