Warfarin may cause bleeding and less clot

In May, Karen Schmale was rushed to Barnes-Jewish Hospital in St. Louis, gasping for air. Diagnosed with blood clots in her lungs, she was given a powerful blood thinner called warfarin.

The medicine probably helped save her life. Then it almost killed her. About a week later, Ms. Schmale, 49 years old, noticed blood in her urine and soon became so weak she could barely climb the stairs to her second-floor apartment. The warfarin was causing the bleeding, and she had to go back to the hospital for an emergency blood transfusion.

A genetic test revealed Ms. Schmale was unusually sensitive to the drug and needed a smaller dose. Before the test, “nobody knew I was going to react like that,” says Ms. Schmale, a data-entry coordinator at a university in St. Louis.