The letter dated Sept. 12 cautions that chemicals with gadolinium, which are used in magnetic resonance imaging procedures, can raise the risk of a sometimes fatal disease that causes excessive tissue to grow between internal organs.

The new warning appears in a black box on the packaging for the chemicals, which are injected into the body to improve quality of images.

The Food and Drug Administration requested the black box, which is the most serious type of warning a medical product can carry, in the spring.

The language warns that patients with kidney failure are more likely to develop a very rare disease called nephrogenic systemic fibrosis when treated with the imaging chemicals. FDA has received more than 250 reports of patients developing the disease after receiving treatment with the chemicals.

The other companies co-signing the letter were Italian firm, Bracco Diagnostics Inc. and St. Louis, Missouri-based Mallinckrodt Inc.