A Cochrane review included 6 studies, most of which were quite old. In comparison of desogestrel vs levonorgestrel progestin-only pill, desogestrel was not associated with a significantly lower risk of accidental pregnancy (RR 0.27, 95% CI 0.06 to 1.19; 1 trial, n=1320). However, desogestrel caused more bleeding problems and discontinuation rate was higher, although neither these differences were statistically significant. The trial (n=97) comparing low-dose mifepristone vs levonorgestrel progestin-only pill found similar pregnancy rates. In a trial comparing two progestin-only and two combined oral contraceptives, the progestin-only pill containing levonorgestrel 30 μg had higher efficacy than did the pill containing norethisterone 350 μg.
Comment: The quality of evidence is downgraded by imprecise results (wide confidence intervals) and by potential reporting bias.