Drugmaker Bristol-Myers Squibb is warning against the use of the antibiotic drug Tequin in diabetic patients after reports of several fatalities, the U.S. Food and Drug Administration said on Thursday.

The company is adding a contraindication warning for diabetics to the drug’s label as well as strengthening its warning that Tequin can cause both low and high blood sugar levels in patients, the FDA said.

The label will include warnings that patients who are older, have renal insufficiency and take other medicine that affects glucose levels can also be at risk, according to the agency.

“The FDA will continue monitoring Tequin’s safety to ensure that its benefits outweigh the risks to patients,” the FDA said in a statement.

Bristol-Myers spokesman Eric Miller said the company’s warning was prompted by reports of “serious cases” of high or low blood sugar among Tequin patients.

“Although most of these cases were reversible, very rare events of (high or low blood sugar) were life threatening and a few resulted in fatal outcomes,” Miller said.

The drugmaker sent letters to doctors about the changes in addition to alerting the FDA, Miller said.

Tequin is typically taken to treat patients with pneumonia, bronchitis, gonorrhea and other infections, such as those affecting the skin, urinary tract or kidneys. The drug had global sales of about $150 million last year, Miller said.

Since Tequin was approved in 1999, there have been “rare cases of life-threatening events” involving patients who took the drug, and while most of those incidents were reversible, there were a few fatal outcomes, the FDA said.

Risks of blood sugar swings were added to the warnings section of Tequin’s label in 2002.