Inhibition ratio (I.R.) and transformation index (T.I.): new indexes to compare the effectiveness and clinical behaviour of modern progestin-only pills (POP)

Progestin-only pills (POPs) have emerged as a crucial contraceptive option for women, particularly those contraindicated to oestrogens. This opinion paper introduces two new indices, the Inhibition Ratio (I.R.) (cyclical and daily) and the Transformation Index (T.I.), to evaluate and compare the efficacy and clinical behaviour of modern POPs. The I.R. quantifies the ratio between the progestin dosage in a POP and the minimum dose required to inhibit ovarian function, providing insights into contraceptive efficacy. The T.I., on the other hand, assesses its clinical impact by considering the ratio between the total progestin dose and the dose required to induce endometrial luteinising changes. Both indices thus offer valuable tools for comparing progestins even at significantly different dosages and regimens, providing information on clinical characteristics and drug effects. The newest formulations of POPs (Desogestrel 28 and Drospirenone 24 + 4) have demonstrated higher I.R. and T.I. in comparison to older versions, indicating significant improvements in contraceptive efficacy and clinical impact with better menstrual cycle control. We believe that using these indices will ensure a more informed and personalised choice of progestin not only for contraceptive purposes but also for therapeutic use in gynaecology. The future goal is to develop other progestins with even more advantageous I.R. and T.I., ensuring the best contraceptive efficacy with fewer side effects, even in women at risk (obese, etc.).