The Food and Drug Administration warned patients Friday that using large amounts of a Johnson & Johnson foot ulcer medication could heighten their risk of dying from cancer.

The FDA added its most serious warning to the company’s cream medication Regranex, which is prescribed to treat severe foot and leg ulcers in diabetics. The new boxed warning states that patients who used three or more tubes of Regranex were five times more likely to die of cancer.

When patients used lower amounts of the cream, the FDA said their chances of developing cancer were no higher than patients who had never used the drug.

The findings come from a long-term study comparing 1,600 patients who took Regranex with 2,800 patients who did not, according to the FDA.

The label advises doctors to use caution when prescribing the drug to patients with malignancies.

Regranex is a man-made version of a substance produced by the human body that helps heal ulcers. Because the drug causes cells to divide more rapidly, the FDA said Johnson & Johnson closely monitored patients for reports of cancer, which spreads through uncontrolled cell division.

The FDA in March said it was reviewing the drug’s links to cancer.

A spokeswoman for Johnson and Johnson said the company cooperated with FDA on the new labeling and is “committed to getting the information out to physicians and patients.”

About 750,000 people have used the drug since it was approved by the FDA in 1997.

Shares of New Brunswick, N.J.-based Johnson and Johnson fell $1.20 to $65.76.