The colon cancer drug Avastin (bevacizumab) carries a serious risk of blood clots and related problems, its manufacturer and the US Food and Drug Administration warned doctors last week.

In a letter to health care providers, Genentech said the drug doubled the risk of problems like stroke, heart attack, and angina in patients who were using it along with standard 5-FU chemotherapy. The conditions may prove fatal, the company warned.

People who have a history of these problems or who are 65 or older are especially at risk. Patients who develop one of these problems during Avastin therapy should discontinue the drug, Genentech said.

Avastin was approved by the FDA in February for people whose colon cancer has spread to other parts of the body. Although blood clots were recognized as a potential side effect even then, ongoing analysis of clinical trials of the drug has helped identify which patients are most likely to be affected, said Genentech spokeswoman Colleen Sweeney.

"It gives doctors more context as they're making treatment decisions and developing a risk-benefit profile," she said.

The company is working with the FDA to determine what new information to include on a revised drug label, she added.

Avastin is a monoclonal antibody that prevents new blood vessels from forming in tumors, essentially starving them of the oxygen and nutrients they need to grow. In studies, the drug was shown to help patients live about 5 months longer than standard chemotherapy alone.

Colon cancer is the third most common cancer among both men and women in the United States. Because procedures like colonoscopy can find this cancer early -- or even prevent it entirely by detecting colon growths before they become cancerous -- the American Cancer Society recommends that adults 50 and older get screened regularly.