Federal health officials said Monday that patients taking a Bristol-Myers Squibb drug for HIV are at risk of a rare, but potentially fatal, liver disorder.

The Food and Drug Administration said it has received 42 reports of the disorder since Videx was approved in 1991. Four patients died from bleeding or liver failure after developing the problem, known as non-cirrhotic portal hypertension.

The problem involves dangerously slow blood flow though the liver, which can cause veins in the esophagus to swell. These veins are thin and can cause burst, causing potentially deadly bleeding.

FDA says it is keeping the drug on the market because its benefits to patients outweigh its risks. Videx capsules prevent HIV from multiplying and are used in combination with other virus-fighting drugs in adults and children.

The agency added warnings to the drug's label about the signs and symptoms of the liver disorder. According to the new labeling, patients "should be monitored for early signs of portal hypertension during routine medical visits."

New York-based Bristol Myers Squibb said the FDA is also requiring the company to design a medication guide for patients that details Videx's risks.

"As with all our medicines, we work closely with the FDA to monitor safety events," the company said in a statement.