Washington (SmartAboutHealth) - Genentech has come ou t to defend their popular cancer Avastin after it was linked to the development of blood clots in the veins.

Researchers this week led by Dr. Shenhong Wu focused on 15 different clinical trials and found that Avastin increased the risk of blood clots in the veins by 33%.

Genentech has quickly responded to this study stating that the warning labels on the drug already make these risks very clear.

They are challenging the study in stating that the results could have been swayed due to a variety of factors.

The study was published this week in the Journal of the American Medical Association.

Avastin is marketed by both Genentech as well as Swiss drugmaker Roche.

The drug is used as a way to stop the growth of cancer tumors.