Vanderbilt University researchers are expressing concern that the diet drug rimonabant (Acomplia) may potentially increase the risk of ectopic pregnancies in young women, according to a report in the October issue of the journal Nature Medicine.

Ectopic pregnancy occurs when a fertilized egg implants in tissue outside of the uterus, and the placenta and fetus begin to develop there. The most common site is within a fallopian tube.

The Vanderbilt researchers, in a study of the CB1 receptor which is targeted by rimonabant, found that in pregnant mice that lacked the gene for the receptor, or in which the receptor was blocked, the embryo failed to go through the oviduct – the tube leading from the ovaries to the uterus.

It is not known whether drugs that block the CB1 receptor can cause ectopic pregnancy in humans.

But the report's senior author, Sudhansu K. Dey, Ph.D., said he was concerned about the potential impact of rimonabant which suppresses appetite by blocking the CB1 receptor.

This drug, if approved, would likely be taken by many young women of reproductive age, Dey said.

"What will happen if they consume anti-CB1 drugs?" asked Dey, Dorothy Overall Wells Professor of Pediatrics and professor of Cell & Developmental Biology and Pharmacology at Vanderbilt University Medical Center.

According to the U.S. Centers for Disease Control and Prevention, about 100,000 ectopic pregnancies occur in the United States each year (out of more than 6 million total pregnancies) and account for about 9 percent of all pregnancy-related deaths in the country. Risk factors include pelvic inflammatory disease, which can scar the fallopian tubes, and smoking.

The study was supported by the National Institute on Drug Abuse and the National Institute of Child Health and Human Development.