Background: The introduction of a new progestin-only oral contraceptive in Europe has renewed interest in this class of oral contraceptives. Unlike the more widely used combined oral contraceptives containing an estrogen plus progestin, these pills contain only a progestin (progestogen) and are taken without interruption. How these pills compare to others in their class or to combined oral contraceptives is not clear.
Objectives: This review examined randomized controlled trials of progestin-only pills for differences in efficacy, acceptability, and continuation rates.
Search strategy: We searched the computerized databases MEDLINE, Cochrane Central Register of Controlled Trials (CENTRAL), POPLINE, LILACS, and EMBASE for studies of progestin-only pills. We also searched for current trials via ClinicalTrials.gov and ICTRP.
Selection criteria: We included all randomized controlled trials in any language that included progestin-only pills for contraception. We incorporated any comparison with a progestin-only pill; this could include different doses, other progestin-only pills, combined oral contraceptives, or other contraceptives.
Data collection and analysis: The first author abstracted the data and entered the information into RevMan 5. Another author performed a second, independent data abstraction to verify the initial data entry. Because of disparate exposures, we were not able to combine studies in meta-analysis.
Main results: Six trials met the inclusion criteria. In the trial comparing the desogestrel versus levonorgestrel progestin-only pill, desogestrel was not associated with a significantly lower risk of accidental pregnancy; the rate ratio was 0.27 (95% CI 0.06 to 1.19). However, the desogestrel progestin-only pill caused more bleeding problems, although this difference was not statistically significant. The trial comparing low-dose mifepristone versus a levonorgestrel progestin-only pill found similar pregnancy rates. In the trial comparing ethynodiol diacetate versus a combined oral contraceptive, irregular cycles occurred in all women assigned to the progestin-only pill (odds ratio 135.96; 95% CI 7.61 to 2421.02). In a trial comparing two progestin-only and two combined oral contraceptives, the progestin-only pill containing levonorgestrel 30 mug had higher efficacy than did the pill containing norethisterone 350 mug. An early trial found megestrol acetate inferior to other progestin-only pills in terms of efficacy. A study of the timing of pill initiation after birth found no important differences, but high losses to follow up undermined the trial.
Authors' conclusions: Evidence is insufficient to compare progestin-only pills to each other or to combined oral contraceptives.