CHICAGO, Aug 16 (Reuters) - U.S. health regulators on Thursday warned doctors of the potential for Bristol-Myers Squibb Co.'s <BMY.N> hepatitis B treatment to lead to resistance to the HIV virus in patients with both diseases.

The U.S. Food and Drug Administration also added a black boxed warning, the strongest caution wielded by regulators, to the labeling of the treatment, known as Baraclude, or generically as entecavir.

In a letter to doctors, Bristol warned that patients co-infected with both the hepatitis B and the human immunodeficiency virus (HIV) should not take Baraclude unless they are also being treated with the standard anti-retroviral therapy. Resistance to the HIV virus can develop if a patient is not being treated for HIV, the warning said.

HIV antibody testing should be offered to any potential patient to get Baraclude, Bristol said. The most advanced stage of HIV infection is AIDS, or acquired immunodeficiency virus.

Hepatitis B is caused by a virus attacking the liver. It can lead to infection, scarring of the liver, liver failure and death. Those at greatest risk are drug abusers, gay men and health care workers.

About 1.25 million Americans are chronically infected, although the number of new infections has been declining, according to the U.S. Centers for Disease Control and Prevention.

Worldwide it is a much graver problem, infecting 2 billion people, according to the World Health Organization. The WHO calls it "one of the major diseases of mankind and ... a serious global health problem."

Bristol reported sales of $59 million from the drug in the second quarter, up from $14 million a year earlier. It also named the drug as a growth driver. (Reporting by Kim Dixon)

AlertNet news is provided by