Birth control pills with two phases versus one phase

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.

Authors' conclusions: 

Conclusions are limited by the identification of only one trial, the methodological shortcomings of that trial, and the absence of data on accidental pregnancies. However, the trial found no important differences in bleeding patterns between the biphasic and monophasic preparations studied. Since no clear rationale exists for biphasic pills and since extensive evidence is available for monophasic pills, the latter are preferred.

Read the full abstract...

Side effects caused by oral contraceptives discourage compliance with, and continuation of, oral contraceptives (OCs). Three approaches have been used to decrease these adverse effects: reduction of steroid dose, development of new steroids, and new formulas and schedules of administration. The third strategy led to the biphasic oral contraceptive pill.


To compare biphasic with monophasic oral contraceptives in terms of efficacy, cycle control, and discontinuation due to side effects. Our a priori hypotheses were: (a) biphasic oral contraceptives are less effective than monophasic OCs in preventing pregnancy; (b) biphasic oral contraceptives cause more side effects, give poorer cycle control, and have lower continuation rates.

Search strategy: 

We searched the computerized databases MEDLINE, POPLINE, LILACS and CENTRAL, as well as clinical trials databases ( and ICTRP). In addition, we searched the reference lists of relevant articles and book chapters. We also contacted the authors of relevant studies and pharmaceutical companies.

Selection criteria: 

We included randomized controlled trials comparing any biphasic with any monophasic OC when used to prevent pregnancy.

Data collection and analysis: 

We examined the studies found during the various literature searches for possible inclusion and assessed their methodology using Cochrane guidelines. We contacted the authors of included studies and possibly randomized studies for information about methodology and outcomes. We entered the data into RevMan, and calculated Peto odds ratios for the incidence of intermenstrual bleeding, absence of withdrawal bleeding, and study discontinuation due to intermenstrual bleeding.

Main results: 

Only one trial of limited quality compared a biphasic and monophasic preparation. The study examined 533 user cycles of a biphasic pill (500 μg norethindrone plus 35 μg ethinyl estradiol (EE) for 10 days, followed by 1000 μg norethindrone plus 35 μg EE for 11 days; Ortho 10/11) and 481 user cycles of a monophasic contraceptive pill (1500 μg norethindrone acetate plus 30 μg EE daily; Loestrin). The study found no significant differences in intermenstrual bleeding, amenorrhea and study discontinuation due to intermenstrual bleeding between the biphasic and monophasic oral contraceptive pills.