Progestin potency – Assessment and relevance to choice of oral contraceptives

ObjectivesTo evaluate current information on the potency of older and newer progestins and the relevance to oral contraceptive (OC) use.MethodsA medline search back to the last review (1985) was conducted.ResultsA thorough review of the pharmacology of the older and newer progestins and their classification is presented. This is followed by a review of the methods of assessment of progestin potency in women. Histological as well as newer biochemical methods (nuclear receptors and placental protein 14) are examined. Progestin potency values for the older and newer progestins are presented and the reasons for the discrepancies discussed. The delay of menses assay and its problems with newer formulations is examined including data on current 30–35 μg ethinylestradiol containing OCs. The role of progestin potency in disease causation and prevention, especially in relation to breast cancer and the protective effects in ovarian and endometrial cancer is reviewed. The term ‘effective progestin activity’ (EPA) which is dose X potency is defined. The EPA enables comparisons of differing progestin containing preparations. The terms ‘low dose’ and ‘low potency’ and their historical introduction and implications are discussed. Newer epidemiological studies still use the old histological data and delay of menses data despite their limited relevance to OCs in present use.ConclusionsNewer progestins are more receptor selective and potency is less relevant than it was with older progestins. Epidemiological studies of progestin potency and its role in disease generally use out of date information. There is still confusion about the relationship of dose and potency in some studies. The use of the EPA can help eliminate this.