| Therapeutic Dosages
| Therapeutic Uses
| Safety Issues
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
Estriol is manufactured in the body from estrone, estradiol, and
. When taken as a drug, it is manufactured synthetically, or extracted from animal products.
The usual dose of estriol is 2 to 8 mg taken once daily. Estriol is also commonly sold in combination with other forms of estrogen.
Like more common forms of estrogen, estriol is used for the treatment of
Double-blind, placebo-controlled studies
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
Estriol might cause less vaginal bleeding as a side effect than other forms of estrogen, although this has not been definitively established.
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 studies.
It is more likely that estriol increases cancer risk in much the same way as other forms of estrogen (see Safety Issues, below).
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.
Another large study found that oral estriol increased the risk of uterine cancer.
In a third study of 48 women, estriol (1 mg twice daily) caused uterine tissue stimulation.
In contrast, a 12-month double-blind trial of oral estriol (2 mg daily) in 68 Japanese women found no effect on the uterus.
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 progesterone. Additionally, one study suggests that estriol is less likely to affect the uterus when taken in a once-daily dose rather than in multiple daily doses.
However, the uterus isn't the only organ at risk of cancer.
Test tube studies
suggest that estriol is just as likely to cause breast cancer as any other form of estrogen.
While this preliminary evidence doesn't constitute proof, it does raise alarm bells. Until proven otherwise, estriol must be regarded as increasing breast cancer risk.
As with other forms of estrogen, vaginal estriol preparations are safer than oral preparations.