Women going through the menopause may suffer hot flushes for as long as 14 years, warn researchers.
They found half the women in a large study had uncomfortable, often distressing symptoms for more than seven years on average.
US experts said greater efforts are needed to find new ways of helping women at the menopause as HRT is currently recommended for five years of maximum use.
Nancy Avis, a professor of social sciences and health policy at Wake Forest Baptist Medical Center, North Carolina, and the study's lead author, said doctors should advise women that vasomotor symptoms such as hot flushes and night sweats might last longer than they had been led to believe.
She said 'The duration of 7.4 years highlights the limitations of guidance recommending short-term hormone therapy and emphasises the need to identify safe long-term therapies for the treatment of vasomotor symptoms.'
In the UK, the average age of the menopause is 51 years, when periods stop and oestrogen hormone levels decline.
The study of 1,449 women with frequent hot flushes or night sweats is the largest study of its kind and included four ethnic groups.
The average length of time women endured symptoms was 7.4 years.
Half of the women were affected for less than that time, but half had symptoms longer - some for at least 14 years, researchers reported in JAMA Internal Medicine (must credit).
Overall, African-American and Hispanic women experienced hot flashes significantly longer than white or Asian women.
The study found that the earlier hot flushes started the longer they lasted.
For women who got hot flushes before they stopped menstruating, they were likely to continue for years after the menopause, longer than for women whose symptoms only began when their periods had stopped.
Researchers found half the women in a large
study had uncomfortable, often distressing
symptoms for more than seven years
In the study, one in eight women began getting hot flushes while still having regular periods.
For two-thirds of women, they began in perimenopause, the period of time before the menopause sets in when periods become more scant.
Women who started getting hot flushes when they were still having regular periods or were in early perimenopause experienced symptoms for around 11.8 years.
About nine of those years occurred after menopause, nearly three times the average of 3.4 years for women whose hot flushes did not start until their periods stopped.
Prof Avis said 'If you start later it’s a shorter total duration, and it's shorter from the last period on.'
The study also found that women with longer-lasting symptoms tended to have less education, greater perceived stress, and more depression and anxiety.
Hot flushes (known as flashes in the US) are caused by dilation of the blood vessels and increased flow of blood to the head and neck, causing reddening of the skin and sweating.
Night sweats are hot flushes that take place at bedtime, disrupting sleep and causing fatigue and stress.
Studies have found that women with hot flush symptoms also face increased risk of cardiovascular problems and bone loss.
Researchers followed the women in the study, who came from seven American cities, from 1996 to 2013. All of them met the researchers' definition for having frequent symptoms: hot flushes or night sweats at least six days in the previous two weeks. None had had a hysterectomy or both ovaries removed and none were on hormone therapy.
The study comprised of women from different ethnic groups living in America, including African-Americans, Japanese and Chinese descent and Hispanic as well as white women and reasons for variation in the length of menopause between the groups were unclear, said the researchers.
Dr JoAnn E. Manson, chief of preventive medicine at the Harvard-affiliated Brigham and Women's Hospital and an author of a commentary on the study, said 'Women with more stress in their lives may be more aware of their symptoms and perceive them to be more bothersome.
'But also having significant night sweats that interrupt sleep can lead to stress' she added.