Prescribing iron supplements may help some women with fatigue even if they are not officially anaemic, Swiss researchers suggest.
A severe shortage of iron is the most common cause of anaemia resulting in lethargy, weakness and feeling faint.
A study of 198 women, published in the Canadian Medical Association Journal, showed iron pills may also help those with moderately low iron levels too.
Taking too much iron, however, can be dangerous.
The researchers looked at reproductive age women, between 18 and 53, as menstruation is known to lower iron levels.
They said unexplained fatigue was a common problem in doctors' surgeries, but it was as yet unknown whether iron supplements could help these women.
Nearly 200 women reporting fatigue, with no medical explanation, took part in the study. Half were given 80mg oral iron tablets every day, while half were given sugar pills.
Medical "fatigue scores" were recorded before treatment and after 12 weeks of taking the medication.
One of the researchers, Dr Bernard Favrat from the University of Lausanne in Switzerland, said: "We found that iron supplementation for 12 weeks decreased fatigue [scores] by 50%.
"Iron deficiency may be an under-recognised cause of fatigue in women of child-bearing age.
"For women with unexplained, prolonged fatigue, iron deficiency should be considered."
Rick Miller, a spokesman for the British Dietetic Association, warned that taking too much iron could result in major organ failure.
He said: "First and foremost, fatigue is multifactorial - it's never just one root cause or micronutrient."
He said fatigue was not fully understood and other factors such as exercise, diet and sleep patterns played a role.
However, he added: "I think the study has some validity, but taking everything into consideration is key."