In a commentary to be published Tuesday in the Journal of Obstetrics and Gynecology Canada, they point to key flaws in a study on the drug Diclectin.
The drug is a combination of an antihistamine and vitamin B6.
Lead author Dr. Nav Persaud says Canadian guidelines that recommend Diclectin for pregnant women who need medication for nausea and vomiting are based on a meta-analysis published in 1997 by researchers at Motherisk Canada, part of the Hospital for Sick Children in Toronto.
A meta-analysis is a study that gathers together and synthesizes the available clinical trial results on a drug or a treatment.
Persaud says that the 1997 meta-analysis contained errors, the result of which is that the evidence supporting Diclectin's safety is not as strong as the meta-analysis authors suggested.
The major flaw is that the drug combination was studied in far fewer women than the meta-analysis suggested, which has the effect of overstating the safety profile, he says.
A family physician and researcher, Persaud says he isn't saying the Diclectin is unsafe, only that the drug does not prevent birth defects, as the 1997 paper claimed.
He suggests the meta-analysis should not be part of the scientific literature and the Canadian treatment guidelines, which drew on the meta-analysis, should be revised.
Diclectin is a combination of doxylamine, an antihistamine, and pyridoxine, which is vitamin B6. It is commonly used in Canada.
Persaud says he now recommends vitamin B6 when pregnant women ask for something they can take for nausea and vomiting.
Dr. Gideon Koren, the senior author of the 1997 paper, was out of the country and not available for comment.
But the chief executive officer of the Society of Obstetricians and Gynecologists of Canada says the flaws Persaud identified in the 1997 paper do not change that organization's position on Diclectin.
Dr. Jennifer Blake says the society's guidelines were drawn up by an expert committee, which reviews the emerging medical literature on an ongoing basis. That group examined Persaud's concerns about the 1997 paper and decided no change was needed at this time, she says.
Blake notes that the U.S. Food and Drug Administration last year approved Diclegis, a drug containing the doxylamine-pyridoxine combination. The regulatory agency gave the drug class A approval, their highest class of safety.
Diclegis is made by Duchesnay Inc., of Blainville, Quebec, the maker of Diclectin.
In a perspective article published last month in the New England Journal of Medicine, FDA officials outlined the rationale for bringing this combined drug back to the U.S. market. (An earlier version, Bendectin, was voluntarily withdrawn by manufacturer Merrell Dow in 1983.)
The FDA officials called the drug "the single most studied pharmacologic therapy for use in pregnancy" and said the available data reveal a favourable risk-benefit profile for Diclegis.