The latest weight-loss drug rimonabant could cause pregnancies to fail, say leading scientists.

Their warning follows new evidence that similar chemicals prevent embryos implanting normally in the womb.

Rimonabant, sold as Acomplia by Sanofi-Aventis, was launched in the UK earlier this summer on a wave of publicity as the drug that can help people lose 10% of body weight in a year by switching-off their appetites. It has also been cleared as a smoking-cessation treatment.

But new research sponsored by the US government suggests it, and other medicines under development that target the ubiquitious CB1 receptor, may have a less desirable effect: altering levels of a key chemical that governs embryo development and implantation.

Even small variations is the level of this chemical, called anandamide, have now been shown to have disastrous consequences for the embryos of pregnant mice.

Prof Sudhansu Dey, the director of reproductive and developmental biology at Vanderbilt University Medical Centre in Nashville said: “I’m very worried that rimonabant will be used extensively by women of reproductive age and I worried about the effective this might have on pregnancies.”

His studies, reported in the Journal of Clinical Investigation, show that chemicals in marijuana smoke can disrupt anandamide levels in pregnant mice and cause their embryos to die. He says rimonabant , which is chemically related, has same potential to disrupt anandamide levels and harm embryos.

Prof Herbet Schuel, an expert in reproductive biology at the State University of New York in Buffalo, said: “Given the results presented by Prof Dey’s study, we need to be very sure that rimonabant doesn’t have unwanted effects on women of reproductive age.”

A spokeswoman for the company said: "Sanofi-aventis does not recommend the use of rimonabant during pregnancy and advises patients who are planning to become pregnant to seek immediate medical advice."