An analysis of 15 clinical trials involving Genentech’s cancer drug Avastin has found that the popular cancer drug was linked to the development of blood clots in the veins. Researchers led by Dr. Shenhong Wu of Stony Brook University Cancer Center in New York, who reported their findings in the Journal of the American Medical Association, said the results of the analysis show a significant increased risk with Avastin for patients while they are taking chemotherapy. According to the analysis, 6 percent of patients taking Avastin had severe blood clots in their veins, while 4 percent of those being treated with chemotherapy were reported with the condition.

Wu said that both patients and doctors should constantly check for signs of blood clots, but they should not stop taking the drug. He said it is important that patients are fully informed of the risks.

Blood clots can be deadly if they travel to the lungs, where it becomes a pulmonary embolism. The blood clot forming in the veins can break loose and travel through the circulatory system to the right side of the heart.

The label warns about the risk of blood clots and mentions the fact that some people taking Avastin and chemotherapy have had blood clots in the veins.

Avastin is marketed by Genentech of South San Francisco and the Swiss drugmaker Roche. It slows the formation of blood vessels that supply tumours using a technique called anti-angiogenesis mechanism, which cuts a tumour’s ability to grow and spread in the body.