Study: Progestin-only pills more effective than previously thought


A literature review published in the journal Contraception finds progestin-only pills (POP) may be more effective than expected.

A literature review published in the journal Contraception finds progestin-only pills (POP) may be more effective than expected.

The review was conducted by Carmela Zuniga, MA, an associate research scientist at Ibis Reproductive Health in Cambridge, Massachusetts, and colleagues.

Researchers looked for relevant studies using the Cochrane database, PubMed, and PubMed Central through March 07, 2022. Fifty-four studies were selected for the final analysis. Zuniga and colleagues said they chose English language articles that had a pregnancy life table rate or a Pearl Index.

“We excluded articles only assessing formulations that: were never marketed globally, are only sold in combination with estrogen, are currently sold only for noncontraceptive purposes, or were not given to participants continuously,” the authors said. They assigned 4 researchers to extract the data independently and 2 to analyze this data using R and Excel.

Study coauthor Zuniga spoke with Contemporary OBGYN® about the results. She said that after reviewing studies published over the course of 5 decades, the research suggests that POPs may be more effective than previously believed.

“Currently, 7 pregnancies are estimated to occur if 100 people took POP for a year. Our findings suggest that the number of estimated pregnancies may be closer to 2,” she said. Zuniga explained that this estimate is for typical POP use, and includes patients who take the pill correctly, those who take it incorrectly, or those who use it inconsistently.

Zuniga told Contemporary OBGYN® that what makes this study significant is unlike previous literature reviews that have focused on few POP formulations or only included information from randomized trials, this review synthesized POP effectiveness and efficacy rates available from a range of study types and POP formulations.

“Since we included nonrandomized studies and studies that analyzed one type of POP as opposed to studies comparing different POP formulations, we assessed each study’s risk of bias and focused our analysis on results from studies assessed to be at low or moderate risk of bias,” she said. She added that the study authors found that the median rate of unintended pregnancy during typical POP use was lower than expected when including all studies, as well as when excluding studies at high risk of bias.

Zuniga said that much of the literature on POP efficacy is on older POP formulations and information comes from studies published 20 to 50 years ago. “Researchers now have more rigorous procedures for collecting data and reporting their results. We recommend that future studies evaluating POP efficacy or effectiveness report Pearl Index rates with confidence intervals, as well as life table rates,” she said. More research is needed to understand if and to what extent patient characteristics impact effectiveness rates for different POP formulations, Zuniga added.

“Decades of research has shown that POPs are a safe and effective contraceptive method, yet only 4% of contraceptive pill users in the United States use POPs,” she told Contemporary OBGYN®. She explained that although the reason for such low rates of use is unknown, it’s possible that views about the effectiveness of POPs may prevent providers from prescribing them.

“There is also a common belief that POPs lose effectiveness when not taken every 24 hours, with a strict 3-hour window. There is little clinical data to support this belief and several studies suggest that this strict timing may not be applicable for some formulations, such as norgestrel, desogestrel, and drospirenone,” she said.

She added that current guidelines and recommendations about POPs need to be revised to reflect the most up-to-date findings.

“Although there are many different POP formulations, only 2 are currently available in the US: northethindrone and drospirenone. The FDA is currently considering an application for an over-the-counter POP product containing norgestrel. As more POP formulations become available to patients in the US, providers can help patients understand the differences between various POPs, as well as between POPs and combined estrogen-progestin oral contraceptives,” Zuniga said.


Zuniga C, Blanchard K, Harper CC, Wollum A, Key K, Henderson JT. Effectiveness and efficacy rates of progestin-only pills: A comprehensive literature review. Contraception. 2022 Dec 17:109925. doi: 10.1016/j.contraception.2022.109925. Epub ahead of print. PMID: 36535414.

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