Health reporter, BBC News
Period pain is a common complaint
Exercise does not help to alleviate period pain, despite it being commonly recommended for women with monthly symptoms, say researchers.
A study of more than 650 university students reported in BJOG found 28% had moderate to severe period pain.
But Birmingham University researchers said they found no link with the amount of exercise the participants did.
GPs said women should be encouraged to do exercise regardless but drugs are available for those with period pain.
The study authors said beliefs about exercise being an effective treatment for bad period pain had persisted for years.
They carried out a questionnaire among 18 to 25-year-olds to find out what age they started their period, how often they had periods, what contraception they used, and whether they had children or had any conditions such as endometriosis or fibroids.
The students were also asked what type of exercise they did and how often as well as other general lifestyle questions.
Responses showed that 72% had no or very little period pain but 28% had moderate to severe pain with their monthly cycle.
After taking into account mood, ethnicity, weight, smoking, and use of the contraceptive pill, they found no link with how much exercise a woman did and whether she suffered from period pain, or how bad her pain was.
Researcher, Dr Amanda Daley concluded that more research was needed before women are told that exercise will reduce of alleviate period pain.
"Anecdotal beliefs that exercise is an effective treatment have prevailed for many years and while it might seem intuitively appealing to promote exercise as a treatment for menstrual disorders, the findings from this study, along with many others, would not support such a view.
"Of course there are many other important health reasons for encouraging women to be physically active and exercise performed in moderation is unlikely to be harmful."
Royal College of GPs chairman Professor Steve Field said women with period pain should do what works for them and exercise might make them feel better in general.
"It is a common problem and people usually self-medicate.
"Some exercise is good for you of course but the main treatment for period pain is the contraceptive pill."