Side effects of birth control pills may keep women from using them as planned. Attempts to decrease side effects led to the two-phase pill. Pills with phases provide different amounts of hormones over three weeks. Whether two-phase pills lead to fewer pregnancies than one-phase pills is unknown. Nor is it known if the pills give better cycle control or have fewer side effects. This review looked at whether two-phase pills worked as well as one-phase pills. It also studied whether women had fewer side effects with these pills.
We did a computer search for studies of birth control pills with two phases versus pills with one phase. We also wrote to researchers and manufacturers to find other trials. We included randomized trials in any language.
We found only one trial that looked at one-phase versus two-phase birth control pills. The study authors did not report all their methods. Many of the women dropped out of the trial, and the authors did not give the reasons. The pills did not differ in any major ways, including bleeding patterns and the numbers of women who stopped using the pills.
This review did not find enough evidence to say if two-phase pills worked any better than one-phase types for birth control, bleeding patterns, or staying on the pill. The one trial report had method problems and lacked data on pregnancies. Therefore, one-phase pills are the better choice, since we have much more evidence for such pills and two-phase pills have no clear reason for use.