Taking vitamin E can significantly reduce the severity and duration of period pain, research suggests.
Vitamin E is available in chemists and health food shops
The condition, also known as dysmenorrhoea, usually affects teenage girls, and can significantly disrupt their education.
The research, by a team from Tarbait Modarres University in Iran, is published in the British Journal of Obstetrics and Gynaecology.
UK experts said the "breakthrough" could help thousands of young girls.
Period pain affects around 40% of adult menstruating women, and over 10% report it severely limits what they can do for up to three days during each cycle.
Just under 280 girls aged 15 to 17 who had reported suffering from period pain took part in the study.
Half were given 200 "international units" (IU) twice a day (a total equivalent to about 270mg), while the rest took dummy pills (placebos), over four consecutive menstrual periods.
They were asked to rate the pain they experienced during each period, and how many painkillers they took.
The amount of blood lost was also monitored.
At the end of the study, researchers found girls who took vitamin E reported the greatest decrease in the intensity of pain which they felt, which was reflected in a significant reduction in the number of painkillers they took.
Those who took the vitamin also experienced less blood loss than the placebo group.
Writing in the British Journal of Obstetrics and Gynaecology, the researchers say: "The use of vitamin E for dysmenorrhoea in adolescent women is attractive because of the marked effect we have demonstrated, coupled with the absence of significant side effects from vitamin E in therapeutic doses."
Mr Peter Bowen-Simpkins, of the Royal College of Obstetricians and Gynaecologists said: "This is particularly exciting because such treatment is readily available over the counter, is free from side effects, avoids the use of hormones or pain relievers and appears to be very effective.
"This may be a breakthrough in a condition affecting thousands of young girls."