A major US government-led study published Tuesday found no link between ovarian cancer and the use of talcum powder in the genital area, citing data from more than 250,000 women.
The paper appeared in the influential Journal of the American Medical Association (JAMA), which also published an editorial praising the research methods and calling the findings "overall reassuring".
For decades, some women have used talcum powder for genital hygiene to absorb odor and moisture - either through direct application or via underwear, sanitary pads, tampons or diaphragms. The practice is more common among older generations.
But its use has grown controversial because of reported cancer risks.
In 2018, US pharmaceutical giant Johnson & Johnson was ordered to pay out US$4.7 billion to 22 women who had claimed that asbestos in the company's talcum powder products caused them to develop ovarian cancer. An appeal is underway.
In the 1970s, concerns arose over possible contamination of the mineral talc with asbestos, which often form alongside each other in nature.
Some studies showed a higher risk of ovarian cancer among users of talcum powder, which was suspected of entering the ovaries via the vagina and uterus.
But the link remained contested because of the overall low number of studies conducted, with some of them criticized for a methodology that introduced recall bias among participants, while others were not statistically conclusive.
The effect is also difficult to isolate because ovarian cancers themselves are rare: only 1.3 percent of all women risk being affected in their lifetimes.
Unprecedented scale
In the new JAMA paper, led by Katie O'Brien of the National Institute of Environmental Health Sciences, researchers synthesized data from four large studies, which encompassed data from a quarter of a million women from 1982 to 2017.
The studies surveyed participants every year or two on a diverse set of questions related to health, including the use of talcum powder.
The hope was that by scaling up the number of participants, it would be possible to discern weak effects with statistical validity, which would have been undetectable on a smaller population.
Among the 252,745 women followed for a median period of 11.2 years, 2,168 developed ovarian cancer.
No statistically significant association was found when comparing women who had used powder and those who had never used it. Nor was any significant association found when comparing frequency or length of use.
But although the study is the largest of its kind with the longest follow-up time, the authors said there were several limitations.
These included that two of the four data sets missed information on frequency and duration of use.
'Competent, careful study'
There are also inherent limitations associated with so-called observational studies of the population at large - as opposed to controlled clinical trials for drugs - including failure to account for all possible external factors.
Kevin McConway, an emeritus professor of applied statistics at Britain's Open University called it a "good, competent, careful study," but added that it could not definitively rule out a link between talc and cancer entirely.
"Most of the risk differences that the researchers examined were not statistically significant," he said. "That doesn't mean that these differences were definitely zero, only that they were small enough so that they could plausibly be due only to random variation."
He concluded that what the research did establish was that if such a risk did exist, it was likely to be very small.
"I'm not a woman, so can't have concerns about my own health in these respects – but if I were a woman, this wouldn't be high on my list of worries."