Women should be cautious about using the herbal remedy ginseng in the early stages of pregnancy, say researchers.
Ginseng is widely used
A team from the Chinese University of Hong Kong has found evidence that one of the main active components of ginseng can cause abnormalities in rat embryos.
Lead researcher Dr Louis Chan said: "Before more information in humans becomes available, women should be cautious about using ginseng in the first three months of pregnancy."
The study is published in the journal Human Reproduction.
The researchers tested the effect of various concentrations of a compound called ginsenoside Rb1 on nine-day-old rats.
They found that embryos exposed to more than 30 micrograms per millilitre of ginsenoside Rb1 showed signs of significant developmental abnormalities, including problems with their heart, eyes and limbs.
At the highest dose of 50 micrograms the problems were even more pronounced. The embryos were significantly shorter in body length and had fewer immature muscle cells.
Dr Chan said the study provided strong evidence that ginsenoside was capable of causing malformations in rat embryos - known technically as a teratogenic effect.
It was also possible that lower doses of compound led to more minor developmental problems which would not always be picked up.
He said: "Although there are numerous reports in the literature concerning the potential benefit of ginseng, much less is know about the potential toxicity and there are no data about its potential effect on the developing human foetus.
"Yet a survey published in 2001 showed that over 9% of pregnant women report using herbal supplements, and in Asia up to 10% have taken ginseng during pregnancy."
Dr Chan said that in many countries herbal medicines such as extract of ginseng are available over the counter. Manufacturers are not required to submit proof of safety and efficacy before marketing.
Ginsenoside Rb1 is only one of the ginsenosides in commercially available ginseng.
More than 20 have been identified and previous studies had shown that different ginsenosides might have different actions.
Dr Chan said: "Although results from animal teratogenicity studies may not reflect the circumstances in humans, our findings suggest that further investigations and monitoring of embryonic effects of ginsenoside on human pregnancy are warranted."
Ginseng is used to enhance stamina and the capacity to cope with fatigue and physical stress.
It is also believed to have an anti-cancer function and to improve cognitive and physical performance, and has shown potential as a treatment for diabetes and obesity.
Some people also recommend it as a treatment for morning sickness.
Dr Chun-Su Yuan, from the University of Chicago, has conducted experiments showing that an extract of ginseng can normalise blood sugar levels in mice bred to have diabetes.
He told BBC News Online: "In general, I think that ginseng, like all other herbal medicines and dietary supplements, has both benefits and potential risks.
"Very likely, the risks could be dose related. The mentality that 'little is good, more is better' could encourage consumers to take a high dose of herbs in an environment with limited dosing guidelines.
"It is very import to address potential risks of any commonly used herbal medicines. However, to translate data from this in vitro animal study to human reproduction, many well-designed studies are needed."