The U.S. Food and Drug Administration has enhanced warnings of side effects of a group of strong antibiotics used to treat a variety of respiratory and urinary tract infections and limited their use to patients with no alternatives.

The antibiotics known as fluoroquinolones include Johnson & Johnson's Levaquin, Bayer's Cipro extended-release tablets and Merck Inc's Avelox.

The FDA added a box warning to the antibiotics in July 2008 to inform users about the increased risk of tendinitis in which the tissue connecting muscle to bone becomes inflamed.

The box warning was updated on Tuesday to highlight the disabling and potentially permanent side effects of the drugs.

The regulator said in May that the side effects of the antibiotics outweighed its benefits.