What is it? Overview Usage Side Effects and Warnings

What is Estriol?

Several forms of estrogen occur naturally in a woman's body. The ovary produces a form named estradiol, which is converted into another important estrogen called estrone. Estriol is yet another form of estrogen metabolized from estradiol, weaker than the other two, but still active.

The estrogen tablets prescribed for menopausal symptoms usually contain estradiol, estrone, or a combination of the two. Some alternative medicine physicians have popularized the use of estriol as an alternative, and there is no doubt that estriol is also effective for symptoms of menopause. However, despite claims that it is safer than other forms of estrogen, the balance of evidence suggests that, in fact, estriol presents precisely the same risks (see Safety Issues below).

Like more common forms of estrogen, estriol is used for the treatment of menopausal symptoms . Double-blind, placebo-controlled studies and other controlled trials have found oral or vaginal estriol effective for symptoms of menopause, including hot flashes, night sweats, insomnia, vaginal dryness, and recurrent urinary tract infections. Estriol may also help prevent osteoporosis . 1 2 3 4 5 6 7 8 Estriol might cause less vaginal bleeding as a side effect than other forms of estrogen, although this has not been definitively established. 9 Some alternative practitioners claim that estriol actually fights cancer, as opposed to estrogen, which increases risk of some cancers. However, this claim is based on exaggerated interpretations of very weak...

Safety Issues

Like other forms of estrogen, oral estriol stimulates the growth of uterine tissue. This leads to risk of uterine cancer. In a placebo-controlled study of 1,110 women, uterine tissue stimulation was seen among women given estriol orally (1 to 2 mg daily) as compared to those given placebo. 10 Another large study found that oral estriol increased the risk of uterine cancer. 11 In a third study of 48 women, estriol (1 mg twice daily) caused uterine tissue stimulation. 12 In contrast, a 12-month double-blind trial of oral estriol (2 mg daily) in 68 Japanese women found no effect on the uterus. 13 It may be that the high levels of soy in the Japanese diet altered the results.

To protect the uterus, estriol—like other forms of estrogen—needs to be balanced with...