It is important to emphasize that this was just one study and only evaluated women who already had heart disease.
Estrogen also has side effects. These include worsening of estrogen-dependent diseases, such as uterine fibroids and endometriosis. Other side effects include breast tenderness and breast enlargement, vaginal bleeding, high blood pressure, nausea, vomiting, headaches, jaundice and fluid retention (edema). In general, the higher the dose, the more benefits and the more side effects are likely to occur.
Premenopausal women produce three biologically active estrogens, estrone (E1), estradiol (E2) and estriol (E3). Estradiol is the most abundant estrogen produced and both estrone and estradiol are potent estrogens. Estriol is considered a weak estrogen. Although little scientific data supports the claim, it has been postulated that estrone is a “bad” estrogen and may be the cause of estrogen’s cancer-causing properties, while estriol is a “good” estrogen and may protect against cancer. Estradiol is probably neutral. Oral estrogens, but not estrogens given by systemic routes (patch, skin cream, vaginal cream, under the tongue), are converted into estrone, with potential negative effects for the patient. Oral estrogens, because they are metabolized by the liver, likely exert
different effects than systemic estrogens, which are not metabolized by the liver.
The most commonly prescribed estrogen is Premarin, which is a conjugated equine estrogen (CEE). CEEs are harvested from the urine of pregnant mares and contain 10 different estrogen components, many of which are converted to estrone.
Another recently recognized difference between oral and systemic estrogens has to do with growth hormone (GH). GH is an important hormone made by the pituitary that stimulates the liver to produce another hormone called IGF-1. GH, via IGF-1, has many beneficial effects, including an increase in energy and sense of well-being. GH, itself, has
some negative effects, including inducing diabetes. It has recently been found that oral, but not systemic estrogen, blocks the effects of GH on stimulating IGF-1 at the liver.