Table 613

contraceptives minimizes the overall dose of hormone delivered. These preparations are designed to more closely simulate estrogen-to-progestin ratios that occur physiologically during the menstrual cycle. Ethinyl estradiol and mestranol are the only two estrogen constituents used for oral contraception in the United States. The use of ethinyl estradiol is favored. Mestranol is inactive until it is metabolized to ethinyl estradiol.

Several progestins are used in combination products. Norgestrel (Ovrette) is a mixture of active and inactive enantiomers; levonorgestrel (Norplant) is the active enantiomer. Levonorgestrel and norethindrone are the most potent synthetic progestins in oral contraceptive preparations.

Inhibition of ovulation is the primary mechanism of the contraceptive action of sequential and combination birth control preparations. Ovulation is prevented by the suppression of the midcycle surge of FSH and LH. Estrogens are most active in inhibiting FSH release, but at high enough doses, they also inhibit LH release. In low-dose combination products, the progestin causes LH suppression. The progestin component is also important in causing withdrawal bleeding at the end of the cycle.

Combination oral contraceptive drugs having the lowest effective concentration of both estrogen and progestin should be prescribed. These preparations are known as low-dose oral contraceptive agents. Adverse effects of both estrogen and progestin are minimized with the use of these agents.

Clinical experience with the low-dose combination drugs indicates that the estrogen-to-progestin ratio is critical in achieving maximum contraceptive activity. In certain combinations (Ortho-Novum 7/7/7, Tri-Norinyl, Tri-Levlen, Triphasil), the estrogen-to-progestin ratio is varied in three phases over the initial 21 days by changing the progestin content of the tablets. An example of the estrogen and progestin doses found in this type of oral contraceptive is shown in Table 61.3.

Progestin-only oral contraceptive formulations consist of a low dose of either norethindrone or norgestrel

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