A study showed about 12 percent of the people taking Avastin developed blood clots in the veins, which is a rate that's 30 percent higher than among patients not taking the drug, USA Today reported.

Because clots are among the leading causes of sickness and death among cancer patients, the study's authors recommended Avastin get the U.S. Food and Drug Administration's strongest warning, known as a "black box" warning.

Avastin is among the new "targeted therapies" billed as a safer and gentler alternative to traditional chemotherapy, USA Today said.

Doctors didn't know the magnitude of Avastin's risks until now because earlier studies were too small to show a trend, co-author Shenhong Wu of New York's Stony Brook University said. Wu says his study results are strong because he pooled results of 15 well-designed trials with nearly 8,000 patients.

A spokeswoman for the drug's maker Genentech said Avastin's label already warns about the risk of blood clots, which can be treated or prevented with blood-thinners, the spokeswoman said.