A "blackbox," the strictest form of warning, is being added to Johnson and Johnson's foot ulcer cream after studies found it increased risk of cancer deaths, the U.S. Food and Drug Administration (FDA) said Friday.

The prescription jell is called Regranex or becaplermin gel, and is used to treat refractory leg and foot ulcers in diabetic patients.

The new warning states that patients who use three or more tubes of the gel over two years are five times more likely to die from cancer than those who do not use it. However, there was no overall increase in cancer incidence based on exposure to becaplermin, the FDA said.

The decision to add the blackbox warning came after reviewing data from a retrospective study that compared cancer incidence and cancer mortality among 1,622 patients exposed to becaplermin and 2,809 controls. The study found a five-fold increased risk of cancer mortality in patients exposed to three or more tubes of the cream.

The new label, which is outlined in a black box, advises doctors to use caution when prescribing the drug to patients with malignancies. The active ingredient in Regranex is a recombinant form of human platelet-derived growth factor, which is applied directly to diabetic foot and leg ulcers that are not healing.

Since the drug causes cells to divide more rapidly, the patients were monitored closely patients for reports of cancer, which spreads through uncontrolled cell division.