BACKGROUND: Side-effects caused by oral contraceptives discourage compliance with and continuation of oral contraceptives. 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. We compared biphasic oral contraceptives with monophasic oral contraceptives in terms of efficacy, cycle control and discontinuation due to side-effects. Our a priori hypotheses were: (i) biphasic oral contraceptives are less effective in preventing pregnancy than monophasic oral contraceptives, and (ii) biphasic oral contraceptives cause more side-effects, give poorer cycle control and have lower continuation rates. METHODS: We searched computerized databases Medline, Embase, Popline and the Cochrane Controlled Trial Register. Additionally, we searched the reference lists of all potentially relevant articles and book chapters. We also contacted the authors of relevant studies and pharmaceutical companies in Europe and the USA. We included randomized controlled trials comparing any biphasic oral contraceptive with any monophasic oral contraceptive when used to prevent pregnancy. We examined the studies found during the various literature searches for possible inclusion and assessed their methodological quality using the Cochrane guidelines. We contacted the authors of all included studies and of possibly randomized studies for supplementary information about the study methods and outcomes. We entered the data in RevMan 3.1, imported the data into RevMan 4.1, and calculated Peto odds ratios for the incidence of intermenstrual bleeding, absence of withdrawal bleeding and study discontinuation due to intermenstrual bleeding. RESULTS: Only one trial of limited quality compared a biphasic and monophasic preparation. This trial examined 533 user cycles of a biphasic pill (norethindrone 500 mug/ethinyl estradiol 35 mug for 10 days, followed by norethindrone 1000 mug/ethinyl estradiol 35 mug for 11 days) and 481 user cycles of a monophasic contraceptive pill (norethindrone acetate 1500 mug/ethinyl estradiol 30 mug daily). The study found no significant differences in intermenstrual bleeding, amenorrhoea and study discontinuation due to intermenstrual bleeding between the biphasic and monophasic oral contraceptive pills. 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.
