Side effects of birth control pills may keep women from using them as planned. Attempts to decrease side effects led to the three-phase pill in the 1980s. Pills with phases provide different amounts of hormones over three weeks. Whether three-phase pills lead to fewer pregnancies than two-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 three-phase pills. We 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 three phases. We also wrote to researchers and manufacturers to find other trials. We included randomized trials in any language.
We found only two trials that looked at two-phase versus three-phase birth control pills. The studies did not have good methods and the authors did not report all their methods. Many women dropped out of the studies, which affects what can be said about the results. One study compared two types of two-phase pills with a three-phase pill. The pills did not differ in any major ways, including the numbers of women who stopped using the pills due to health problems. The other trial compared a two-phase pill with two different three-phase pills. The two-phase pill had worse bleeding patterns than the three-phase pill with a different hormone (levonorgestrel). In contrast, bleeding with the two-phase pill was like that of the three-phase pill with the same hormone (norethindrone). The type of hormone may be more important than the phases for cycle control.
These trials did not provide enough evidence to say if three-phase pills worked any better than two-phase types for birth control, bleeding patterns, or staying on the pill. More research would be needed to show whether three-phase pills were better than two-phase pills. However, two-phase pills are not used enough to justify further research.