Genentech

(DNA)

shares fell Friday on concerns about its new colorectal cancer drug Avastin, according to media reports.

The Food and Drug Administration warned that the drug increased the chances of deadly blood clots in arteries.

Shares slid $3.61, or 7.7%, to $43.50, in heavy trading. More than 7.4 million shares have been traded, compared with the daily average of 4.1 million.

Avastin was approved by the FDA Feb. 26 but Genentech is continuing research to secure approval for using the drug to treat other cancers, including breast, lung, kidney and pancreatic forms.

Analysts expect the drug to be a major revenue earner for the company.