Researchers warned gonorrhea is becoming increasingly resistant to antibiotics, which raises the risk of an untreatable strain of the sexually transmitted disease emerging, HealthDay News reported Thursday.

"We're trying to stay a step ahead by putting these warnings and alerts out," said Dr. Gail Bolan, of the U.S. Centers for Disease Control and Prevention's division of sexually transmitted diseases prevention.

Gonorrhea is the second most common STD in America, following chlamydia, with more than 600,000 people contracting it annually.

While drug-resistant gonorrhea isn’t a new phenomenon—resistant strains emerged during World War II, the 1980s and again in 2007—the problem today is that the third-generation cephalosporins used to treat the disease are the only remaining types of effective antibiotics recommended by the CDC.

And now, researchers reported in the New England Journal of Medicine, the STD is showing resistance to these drugs.

"The point now, is that we are down to the last class of antibiotics that we know—that have been studied—to be effective in the treatment of gonorrhea," Bolan said, adding that if a new resistant strain were to spread, “we have the potential of having no other antibiotics to turn to.”

Cephalosporin-resistant strains have been previously identified in Japan and the U.K., though the drugs still remain highly effective against most strains in the United States.

Patient vigilance and coordinated public health policies are needed for the U.S. to avoid the spread of cephalosporin-resistant strains, the researchers said.

Gonorrhea is preventable with the use of condoms, Bolan said, but because the disease is often symptomless, it is also important to undergo routine medical examinations.

"And certainly if you've got symptoms, we're concerned that you . . . get evaluated quickly," Bolan added.

For women, symptoms include vaginal discharge, a burning sensation during urination and pain during intercourse. Symptoms for men include clear penile discharge, burning on urination and pain during defection.

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