A drug prescribed to pregnant women to reduce the risk of early labor could be doing patients more harm than good, a British charity said on Friday.

Tommy’s, a charity that funds national research programs to prevent miscarriage and stillbirths, said the antibiotic metronidazole may double the risk of pre-term deliveries.

“Clinicians and high-risk pregnant women should be aware of this research outcome so that we can avoid escalation of pre-term birth and in turn, save more babies’ lives,” said Professor Andrew Shennan, of St Thomas’ Hospital London and professor of obstetrics for the charity.

An estimated one in 200 pregnant women in Britain are given the drug to treat an infection called bacterial vaginosis that can cause early labor and pre-term delivery.

Women are screened for the infection in the later stages of pregnancy.

According to a study published in BJOG: An International Journal of Obstetrics and Gynaecology, Shennan and his colleagues studied 900 pregnant women who had a high risk of early labor. The women were randomly divided to receive the drug or a placebo.

The scientists said 39 percent of women in the placebo group had a pre-term delivery compared to 62 percent of women who had received metronidazole.

“Preterm delivery may be increased by metronidazole therapy,” Shennan said in the report.

He added that there are some serious conditions for which the drug, a common antibiotic that has been used for decades, is given but it should not be recommended to prevent early labor and pre-term delivery.

“There are other antibiotics which could treat the condition effectively,” Shennan added.