Oral contraceptives in the United States: trends in content and potency.

Data from the National Prescription Audit, a nationwide pharmaceutical marketing research data base, were reviewed to study changes in the type of oral contraceptives marketed in the US, their content and their relative oestrogen and progestin potency over the 21-year time period of 1964-84. Three major types and 42 brands of oral contraceptives were marketed in the US during this time period. All oral contraceptives dispensed in the US have contained one of nine different progestins and one of two oestrogens or, in the case of the progestin-only pills, no oestrogen at all. A comprehensive classification listing all 42 brands of oral contraceptives by content and strength is presented. Secular changes in oral contraceptive potency are described in terms of a categorization scheme which simultaneously ranks both the oestrogen and progestin potencies of each oral contraceptive. Over the time period studied, oral contraceptives have evolved from high strength, high potency drugs to much lower strength, lower potency drugs. The epidemiological implications of these shifts in oral contraceptive content and potency are discussed.