Grapefruit interacts with dozens of drugs, raising levels of the drugs in the bloodstream. It also causes changes in a woman's estrogen levels. The chemical in grapefruit does not interact directly with medicines or estrogen. Instead it binds to an enzyme in the body and reduces absorption. That leads to more of the medication or estrogen passing into the bloodstream, according to Harvard Medical School. As of 2010, research into grapefruit's impact on estrogen levels and possible long-term effects such as increased breast cancer risk was still ongoing.
Grapefruit affects the form of estrogen called estradiol, which the National Institutes of Health deems the most important form of estrogen in a woman's body. Estradiol occurs naturally and is responsible for Fallopian tube, uterus and vaginal growth. It also has a role in body fat distribution. Estradiol also can be made synthetically and used in birth control pills and hormone replacement therapy. It is sometimes administered to prevent bone loss as well, according to the University of Washington in Seattle.
Grapefruit may cause a woman who takes birth control pills or utilizes hormone replacement therapy to higher than desired levels of estrogen in her bloodstream, according to Graedons' Guide to Grapefruit Interactions. The higher estrogen levels can lead to side effects including breast tenderness and nausea. Long-term exposure to high estrogen levels is associated with an increased breast cancer risk.
A 2007 study on postmenopausal women published in the British Journal of Cancer indicates that eating a whole grapefruit daily can raise breast cancer risk because the fruit inhibits CYP3A4. This is the isoemzyme that metabolizes estrogen. This finding is only preliminary, according to the American Cancer Society, so there's not enough evidence to lead the ACS to recommend that women who want to lower breast cancer risk quit eating grapefruit. Further research into the topic is needed, though at least two other studies have concluded that estrogen levels in women who drink grapefruit juice or eat grapefruit are higher.
Studies have yet to detail how long grapefruit's effect on estrogen lasts in a woman's body, according to ACS. Other research on grapefruit interactions has shown that one glass of grapefruit juice can elevate levels of some drugs for as many as 72 hours.
Products for hormone replacement therapy are required by the U.S. Food and Drug Administration to carry warnings about grapefruit consumption. The labels must tell purchasers that grapefruit juice can raise plasma concentrations of estrogen.