Non-prescription pain-relieving drugs generally work well and safely when taken as directed. But physicians say too much of a good thing can have dangerous consequences.

An estimated 125 million Americans regularly take over-the-counter pain relievers. The most commonly used are aspirin, acetaminophen and ibuprofen.Overuse of ibuprofen has recently been linked to serious health problems, including kidney failure in people with mild kidney disease.

Dr. Ervin A. Gombos, a kidney specialist at New York University Medical Center, said there is little risk for most people when the drug is used for a short time.

"However, one should not take even a normal dosage of ibuprofen for more than several days in a row and assume with certainty there will be no harm," Gombos said. "The lowest dosage that provides relief is the correct dosage. If pain is of such intensity and duration that pain medication gives no relief, then medical evaluation is required."

The U.S. Food and Drug Administration is making many more pain relievers available without prescription, thus heightening concern over the effects of their overuse.

"These drugs are generally safe to use without medical supervision insofar as they are used as according to directions," said Dr. Thomas Kantor, a rheumatologist. "But their availability makes unintentional over-medication possible."

Ibuprofen, which has been available without prescription since 1985, is sold under a variety of trade names, including Advil, Medipren and Nuprin. Previously, it was available only as a prescription drug, marketed as Motrin.

Like aspirin and unlike acetaminophen, ibuprofen is a non-steroidal anti-inflammatory drug. It is used in the treatment of a broad range of ailments, including arthritis, menstrual pain, headaches and fever.

"Ibuprofen works by interfering with the body's production of prostaglandin, a potent substance involved in inflammation," Kantor explained. In doing so, ibuprofen may also restrict blood flow throughout the body, including to the kidneys.