RESUMEN
We compared the efficacy of 4 mg drospirenone (DRSP) progestin-only pills (POPs) versus combined oral contraceptive pills (COCs) containing 0.02 mg of ethinyl estradiol (EE) and 0.075 mg of gestodene (GS) in ovulation inhibition and inducing unfavorable cervical mucus changes using a delayed-starting approach. This randomized controlled trial involved 36 participants aged 18-45 years. The major outcomes included ovulation inhibition assessed using the Hoogland and Skouby score, and cervical mucus permeability, assessed using the modified World Health Organization score. The results demonstrated ovulation inhibition rates of 77.8% for the EE/GS group and 88.9% for the DRSP group. The risk ratio and absolute risk reduction were 0.50 (95% confidence interval [CI]: 0.10, 2.40) and - 0.11 (95% CI: - 0.35, 0.13), respectively, satisfying the 20% non-inferiority margin threshold. The median time to achieve unfavorable cervical mucus changes was comparable between the DRSP (3 days, interquartile range [IQR]: 6 days) and EE/GS (3.5 days, IQR: 4 days) groups. However, the DRSP group had a higher incidence of unscheduled vaginal bleeding (55.56% vs. 11.11%; p = 0.005). DRSP-only pills, initiated on days 7-9 of the menstrual cycle, were non-inferior to EE/GS pills in ovulation inhibition. However, they exhibited delayed unfavorable cervical mucus changes compared to the standard two-day backup recommendation.Clinical trial registration: Thai Clinical Trials Registry (TCTR20220819001) https://www.thaiclinicaltrials.org/show/TCTR20220819001 .