WASHINGTON (AP) — Consumer advocates are calling on the Food and Drug Administration to add stronger warning labels about risks for pregnant women related to a constipation drug. The FDA has approved Amitiza to treat chronic constipation and irritable bowel syndome, and it was prescribed more than 1 million times last year. The drug is marketed by Sucampo Pharmaceuticals Inc. and Takeda Pharmaceuticals Co. The label warns that the pills have not been studied in pregnant women and that mothers-to-be should use the drug only "if the potential benefits justify the potential risk to the fetus." But Public Citizen says that note isn't good enough. The consumer watchdog group filed a petition with the FDA on Wednesday demanding the drug receive a "black box" warning, the strongest type available. The group's petition points out that a chemically similar drug called misoprostol carries the boxed warning, stating that it can cause "abortion and premature birth." That drug, marketed by Pfizer Inc. as Cytotec, is approved to treat gastric ulcers, but is used off-label to hasten labor or cause an abortion, according to Public Citizen. Although Amitiza has not been studied in pregnant women, Public Citizen's Dr. Sidney Wolfe says company studies in guinea pigs showed a 4 percent rate of abortion at the equivalent human dose that would be taken for constipation. Animal studies are not always applicable to humans, though they are used as an indicator of toxicity. "The current label is grossly inadequate as it lacks useful information pertaining to the risks of drug-induced abortion in pregnant women who take this drug," said Wolfe in a statement.