The hot flashes that are common during and after menopause may last an average of more than 10 years, more than twice as long as previously assumed, according to a U.S. Study.

The research, published in "Obstetrics and Gynecology," also found that women who start getting hot flashes before menopause or in the early stages of menopause will have them for longer, on average, than women who don't have their first hot flashes until later.

"Hot flashes are pretty common, they're distressing and bothersome to a fair number of women, and they're starting earlier" than previously thought, said Ellen Freeman, at the University of Pennsylvania School of Medicine and the study's lead author.

Freeman and her colleagues followed a group of about 400 women in their 30s and 40s, starting in 1995. Over the next 13 years, researchers interviewed the women every year or so, asking them questions about their health, including menopause symptoms.

Only 55 of the women were entirely free of hot flashes during the study. Another 90 said they had only mild flashes, while the rest -- 259 women -- reported moderate to severe hot flashes in at least one interview.

For women who reported hot flashes, those symptoms lasted an average of 11 and a half years. Moderate to severe hot flashes specifically went on for about 10 years on average.

And they stuck around longer in women who had their first hot flashes when they were younger and before menopause started. Black women and normal weight women also reported having moderate to severe hot flashes at more of the annual interviews than white women and obese women.

The researchers said they didn't know why there was so much variation in the timing of women's first hot flashes in relation to menopause in this study. But one thing does seem clear: they can go on a long time.

"The assumption has been that (hot flashes) last about 4 to 5 years," said Rebecca Thurston, who has studied menopause symptoms at the University of Pittsburgh and was not involved in the latest study.

"Part of the issue has been that we haven't really followed women for long enough or gotten them soon enough" to estimate the duration of the symptoms, she told Reuters Health.

The findings raise the question of what are the best treatments for hot flashes, given that these treatments might be needed for years on end, exports.

Noting that some women do find their own ways of coping, Thurston added: "We need more safe and effective treatments for women that they can use for the long haul."