A heart attack or stroke may be an early sign of cancer.

Researchers studied records of 374,331 Medicare beneficiaries, mean age 76, who were given cancer diagnoses from 2005 to 2013. They matched them with an equal number of controls without cancer. Then they retrospectively tracked heart attacks and strokes in the two groups in the year before the cancer diagnosis.

In the first seven months, there was no difference between the two groups. But from then on, the risk of a cardiovascular event rose in patients who would later be diagnosed with cancer. At one month before diagnosis, those with a cancer diagnosis had more than five times the risk of heart attack or stroke compared with those without a cancer diagnosis.

The researchers found that the highest risks were in those with diagnoses of lung and colorectal cancers. It may be that cancer disrupts the body’s blood system well before the disease is detectable, causing clots that lead to cardiovascular events.

The study, in the journal Blood, had no data on the severity of the heart attacks and strokes, and the authors acknowledge that the results may not apply to younger patients.

“I don’t want to overstate the absolute risk connecting cancer to heart attack and stroke,” said the lead author, Dr. Babak Navi, an associate professor of neurology at Weill Cornell Medicine, “but there is a substantial relative risk connection that has immediate clinical implications.”

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