Italian researchers have found that a synthetic variation of vitamin A sharply cuts the risk of women - especially younger women - getting a second bout of breast cancer.
The substance, called fenretinide, reduces the risk of recurrence in pre-menopausal women by 38%, top cancer researcher Umberto Veronesi said.
In older women the reduction was 17%, he said.
"The younger the woman is, the greater the effect", said Veronesi, Italy's top oncologist and a former health minister.
"This evidence will spur fresh experiments on young momen at high risk of cancer."
"The magnitude of the protective effect is so marked that there is reason to believe this effect is real and replicable in new clinical studies".
The protective effect lasts some 15 years even if the substance is only taken for five years after their tumour is removed, Veronesi said.
The team is hoping to get funding for major trials across Europe and America.
The study, which monitored some 3,000 women over 15 years, appears in the current edition of the Annals of Oncology. Breast cancer is the most common tumour affecting women in developed countries.
Every woman is at risk for breast cancer. The lifetime risk of getting breast cancer is about one in eight. The lifetime risk of dying from it is one in 28. It is the second leading cause of cancer death behind lung cancer.
