(Reuters Health) - More than two-thirds of cannabis dispensaries recommend that pregnant women use marijuana to treat morning sickness even though doctors advise against it because of safety concerns, a U.S. study suggests.

While cannabis and other drugs are not tested in pregnant women to avoid any unintended harms to mothers and babies, animal research to date has linked marijuana use during pregnancy to an increased risk of underweight and premature infants as well as neurological defects.

The American College of Obstetricians and Gynecologists discourages doctors from prescribing or suggesting the use of marijuana for medicinal purposes while women are trying to conceive, pregnant, or nursing their babies.

“Given the concern for fetal harm with maternal cannabis use, women should utilize nausea medications prescribed by a physician for treatment of nausea and vomiting in pregnancy, and should not use cannabis for this,” said senior study author Dr. Torri Metz of the University of Colorado School of Medicine in Aurora.

“First line medical therapy for treatment of nausea and vomiting in pregnancy is vitamin B6 and doxylamine,” Metz said by email. “This combination has been studied extensively and there is not concern for fetal harm, and there are numerous other agents that can also be prescribed if the first line therapy fails.”

As more U.S. states legalize marijuana, concern is mounting in the medical community that many people including pregnant women may mistakenly assume that using the drug is risk-free, researchers note in Obstetrics & Gynecology.

For the current study, researchers posing as pregnant women with morning sickness called 400 randomly selected cannabis dispensaries in Colorado, one of about 30 U.S. states that have legalized some form of marijuana sales. Overall, 69 percent of the dispensaries recommended cannabis to help pregnant women relieve this nausea, the study found.

Medical marijuana dispensaries were even more likely than other cannabis retailers to recommend the drug to pregnant women: 83 percent of medical dispensaries did this, compared to slightly more than 60 percent of other dispensaries.

Most of the people women spoke to at the dispensaries recommended cannabis for use in pregnancy based on their personal opinion, and 36 percent stated that the drug is safe in pregnancy.

While 82 percent of the dispensaries ultimately advised women to speak to a healthcare provider, only 32 percent made this recommendation without prompting.

One limitation of the study is that it’s possible mystery callers might get different advice by phone than pregnant women might get in person at dispensaries, the study team acknowledges.

Still, the results are concerning because medical marijuana laws prohibit dispensing cannabis to pregnant women, said Dr. Nora Volkow, director of the National Institute on Drug Abuse in Bethesda, Maryland.

“One issue is that dispensaries may not have the proper training, and how women got the prescriptions is another issue because this should not be prescribed for pregnant women,” Volkow, who wasn’t involved in the study, said in a telephone interview.

Another problem is that scientists don’t know how different types and amounts of cannabis use might impact pregnant women and their babies,” said Kelly Young-Wolff, a researcher at Kaiser Permanente Northern California in Oakland who wasn’t involved in the study.

“The health effects of cannabis use in pregnancy may differ depending on mode of administration (e.g., vaping versus smoking), cannabis potency, trimester of cannabis exposure, and concurrent use of other substances, factors that we are just beginning to include in research on the health effects of cannabis use in pregnancy,” Young-Wolff said by email.

While cannabis can indeed ease nausea, most of the research in this area has focused on cancer patients who have nausea as a side effect of chemotherapy, said Marcel Bonn-Miller of the Perelman School of Medicine at the University of Pennsylvania in Philadelphia.

“I would not advise pregnant women to use cannabis for nausea, especially because we already have a number of good treatment options,” Bonn-Miller, who wasn’t involved in the study, said by email. “There are just too many risks and unknowns at this stage.”