WASHINGTON (AP)—Federal drug safety officials have imposed the government’s most urgent warning on Cipro and similar antibiotics, citing risks that they can cause tendon ruptures, a serious injury that leaves some patients incapacitated and needing extensive surgery..

The Food and Drug Administration on July 8 ordered makers of flouroquinolone drugs—a potent class of antibiotics—to add a “black box” warning to their products, which include Cipro, Levaquin, Floxin and other medications.

Patients should immediately stop taking the medications if they develop any tendon pain, swelling or inflammation.

The two leading drugs covered by the warning are Cipro, made by Bayer, and Levaquin, which is made by Ortho-McNeil. Cipro became a household name during the anthrax attacks of 2001. In everyday medicine, Cipro is often used to treat urinary tract infections. Levaquin is generally used to treat respiratory infections.

Tendon ruptures are normally thought of as sports injuries, generally occurring usually among men in their mid-thirties. The link to treatment with the antibiotics is highly unusual, and scientists still don’t fully understand why it happens. However, FDA officials stressed that many of the serious injuries appear to be preventable if patients stop taking the drug at the first sign of pain or swelling in a tendon, call their doctor, and switch to another antibiotic.

The FDA’s analysis found that patients with the highest risk include people over 60, those with kidney, heart and lung transplants, and those also taking steroids.