Both Prempro and Premphase are one-tablet hormone replacement therapies available in convenient calendar packs marked with the days of the week.

Although Prempro and Premphase contain the hormones estrogen and progesterone, the dosage is different for each therapy. Prempro contains a smaller amount of estrogen and progesterone (2.5 mg of each); it should be taken on a daily basis throughout the year. Premphase contains larger amounts of both hormones (5.0 mg of each). Subjects taking Premphase alternate between a 14-day regimen of just estrogen, followed by a 14 day regimen of estrogen and progesterone.

Prempro and Premphase are the only one-tablet hormone replacement therapies indicated to help prevent osteoporosis in women at risk and to treat moderate to severe vasomotor symptoms (such as hot flashes and night sweats) associated with menopause.

Most studies have not shown a higher risk of breast cancer in women who have used estrogens. Some studies have reported that breast cancer developed more often in women who used estrogens for long periods of time or who used high doses for shorter time periods. The effects of added progestins on the risk of breast cancer are unknown. Some studies have reported a moderately increased risk in those taking combination estrogen/progestin therapy, even higher than the possible risk of taking estrogens alone. Other studies have not shown this relationship. The addition of progestin may have unfavorable effects on blood sugars and fats, which might make a diabetic condition worse.

The most commonly reported side effects in the clinical study were breast tenderness, headache, and abdominal pain. Rarely, however, were these serious enough to cause women to stop therapy.

Previously, physicians have prescribed estrogen and progestin for nonhysterectomized women needing hormone replacement therapy (HRT) in a variety of different regimens. Until now, a woman taking estrogen and progestin at different times throughout the month had to keep track of where she was in her cycle and whether she needed to take one tablet (estrogen alone), or two tablets (estrogen and progestin) on any given day. As a result, many women found their HRT regimens difficult to follow.

Some women should not take HRT. A woman should make sure her doctor or health care provider is aware of her personal or family health history before taking HRT. This history should include instances of breast cancer, breast lumps, abnormal vaginal bleeding, abnormal blood clotting, severe headache, dizziness, and liver disease. Women who are pregnant should not take HRT. Estrogens have been reported to increase the risk of cancer of the uterus in postmenopausal women.