A commentary on progestin-only pills and the "three-hour window" guidelines: Timing of ingestion and mechanisms of action.

Clinical guidelines for progestin-only pills (POPs) state that each pill should be taken at the same time each day, with only a "three-hour window" of tolerance before back-up contraception should be used. In this commentary, we summarize studies examining the timing of ingestion and mechanisms of action for various POP formulations and dosages. We found that different progestins have different properties that determine the effect of delayed or missed pills on effectiveness at preventing pregnancy. Our findings highlight that there is more margin for error for some POPs than guidelines suggest. The three-hour window recommendation should be re-evaluated in light of these findings. Since clinicians, potential POP users, and regulatory bodies rely on current guidelines to make decisions about POP use, a critical evaluation and update of these guidelines are urgently needed.