The antidepressant Paxil was approved for sale in 1992, the cholesterol-lowering drug Pravachol in 1996. Company studies proved that each drug, on its own, works and is safe. But what about when they are taken together?

By mining tens of thousands of electronic patient records, researchers at Stanford University quickly discovered an unexpected answer: people who take both drugs have higher blood glucose levels. The effect was even greater in diabetics, for whom excess blood sugar is a health danger.

The research is an example of the increasing ease with which scientists now scour digitized medical results, like glucose tests and drug prescriptions, to find hidden patterns. "You're not constrained by the need to actually get patients lined up in a clinical trial that would be incredibly expensive," says Russ Altman, director of Stanford's Biomedical Informatics Training Program, whose group published the Paxil/Pravachol result in the journal Clinical Pharmacology and Therapeutics this July. "We had most of this paper done probably in a month."

The spread of electronic patient records, with their computer-readable entries, is opening new possibilities for medical data mining. Instead of being limited to carefully planned studies on volunteers, scientists can increasingly carry out research virtually by sifting through troves of data collected from the unplanned experiments of real life, as preserved in medical records from scores of hospitals.

Such techniques are allowing researchers to ask questions never envisioned at the time of a drug's approval, such as how a medicine might affect particular ethnicities. They are also being used to uncover evidence of economic problems, such as overbilling and unnecessary procedures. Mining of health records "is going to build advancements in research, but also efficiencies in the health delivery system," says Margaret Anderson, executive director of FasterCures, a think tank in Washington, D.C.

Some large hospital systems that use electronic records now employ full-time database research teams. Laurence Meyer, associate chief of staff for research at the Salt Lake City Veterans Administration Medical Center, says he knows of more than 100 research projects using electronic records from the VA's six million patients, who are seen at 152 hospitals and 804 outpatient clinics across the country.