WASHINGTON – The government and Genentech are warning doctors that Avastin, used to treat colorectal cancer, increases patients’ risk of heart ailments including chest pain, strokes, ministrokes and heart attacks.

Avastin also raises the risk that patients might die from those heart ailments.

Colleen Sweeney, Genentech spokeswoman, said Friday that the heart ailments had killed some patients who took the drug, but she could not say how many. “I don’t have the data,” Sweeney said. Genentech developed and manufactures the drug.

During clinical trials before the drug’s approval, one patient died but a “clear relationship” could not be determined between Avastin use and the heart-related fatality, she said. Additional analyses led to the drug warning by the company and the Food and Drug Administration.

“Right now, we’re working with the FDA, with these analyses, to update the Avastin label,” she said.

Shares in Genentech fell $2.88, or more than 6 percent, to close at $44.23 on the New York Stock Exchange.

Genentech would not say how many patients were prescribed Avastin after FDA approval. “We don’t give out prescription information,” Sweeney said.

The company told health care providers in its warning letter that patients whose cancer had spread were twice as likely to suffer serious heart ailments if they received Avastin with their chemotherapy. The highest risk individuals had a history of such heart problems and were older than 65.

The highly anticipated drug’s progress through clinical trials was watched closely. The South San Francisco-based company’s stock leapt after reports last summer that Avastin helped the sickest colon-cancer patients survive a few months longer than those receiving standard care.

On Feb. 26, the FDA approved the product, a genetically engineered version of a mouse antibody that contains mouse and human components, after testing showed patients who received the drug intravenously lived an additional five months. The drug, the first approved angiogenesis inhibitor to treat colorectal cancer, works by blocking new blood vessels, essentially depriving cancer cells of the oxygen and nutrients required to thrive.

The company’s Aug. 12 warning letter points to the need for more basic research, said Dr. Antonio Giordano, a gene therapy expert and director of the Sbarro Institute for Cancer Research and Molecular Medicine at Temple University in Philadelphia.

“Clearly, these biological drugs are being developed to be specific for the colon. There is a possibility some of the targets in the colon are represented, also, in other tissue and organs,” Giordano said. “I think more specific work needs to be done in order to understand the precise mechanism of the action of this drug.”