A strain of gonorrhea that is resistant to drugs has been found in England, prompting the Chief Medical Officer to issue a warning against "suboptimal treatment" for the sexually transmitted disease (STD).

"Antimicrobial resistance (AMR) is a growing clinical and public health issue of the greatest importance," wrote Chief Medical Officer Dame Sally Davies in a letter addressed to all general practitioners and pharmacies. "It has long been recognized that gonorrhoea is at risk of becoming an untreatable disease due to the continuing emergence of AMR."

The strain of gonorrhea that exhibits resistance to drugs, often referred to as "super gonorrhea," caused an outbreak earlier this year that sparked a nationwide alert.

Last year, antimicrobial resistance was rare for gonorrhea. However, in 2015, an outbreak involving resistance to high level azithromycin was detected in Leeds, threatening the present front line dual therapy treatment for the disease. Resistance to three classes of antibiotics — penicillin, tetracyclines and ciprofloxacin — were also detected, according to the British Association for Sexual Health and HIV.

There have been 16 known cases of "super gonorrhea" since March, reported the Guardian. Fourteen of these were found in heterosexuals.

The disease is commonly treated with a dual therapy of ceftriaxone and azithromycin. However, some pharmacies only offer oral medication, which allowed the organism to develop resistance and caused "super gonorrhea" to emerge.

Public Health England recommends dual therapy because the organism will unlikely develop resistance to both drugs at the same time. Davies emphasized the same principle in her letter.

"Since 2011, the approach in the U.K. and in other countries has been to recommend the use of dual therapy of injectable ceftriaxone and oral azithromycin," she wrote. "Gonorrhoea has rapidly acquired resistance to new antibiotics, leaving few alternatives to the current recommendations. It is therefore extremely important that suboptimal treatment does not occur."

Gonorrhea is the second most common STD in England. In 2013, there were 29,419 reported cases of gonorrhea. In 2014, there were 34,958, showing a 19 percent increase from the previous year, according to Public Health England.

If untreated, gonorrhea can cause pelvic inflammatory disease, which may lead to tubal infertility.