Avastin, Genentech’s best-selling drug, increases the risk of people developing blood clots, according to a study published in the Journal of the American Medical Association.

For the study, researchers looked at data from roughly 8,000 cancer patients who participated in 15 clinical trials of the life-extending cancer medicine Avastin. The findings showed that high and low doses of the drug augmented risk of blood cloth equally.

An estimated 12 percent of patients taking Avastin developed blood clots in the veins. This figure is approximately 30 percent higher than in other cancer sufferers who are not taking the drug.  

According to Kristina Becker, a spokeswoman for Genentech Inc., the label of Avastin already contains a warning about the risk of blood clots.

"I still think it's a useful treatment," lead researcher Shenhong Wu, a cancer specialist at Stony Brook University in Stony Brook, N.Y., said during a telephone interview. "But for some patients with a high risk of clotting, you may have to evaluate the risks against the potential benefits."

Avastin received the U.S. Food and Drug Administration approval as a first-line treatment in combination with chemotherapy for patients without the squamous form of non-small cell lung cancer. The drug was also approved for use in breast and colon cancers.

Researchers who carried out the study said Avastin should get a “black box” warning from the Food and Drug Administration, since blood cloths put the lives of patients at risk.