Category C. Rifampin has been shown to be teratogenic in rodents given oral doses of rifampin 15 to 25 times the human dose. Although rifampin has been reported to cross the placental barrier and appear in cord blood, the effect of RIFADIN, alone or in combination with other antituberculosis drugs, on the human fetus is not known. Neonates of rifampin-treated mothers should be carefully observed for any evidence of adverse effects. Isolated cases of fetal malformations have been reported; ... there are no adequate and well-controlled studies in pregnant women. Rifampin should be used during pregnancy only if the potential benefit justifies the potential risk to the fetus. Rifampin in oral doses of 150 to 250 mg/kg produced teratogenic effects in mice and rats.