A Cochrane review «»1 «Edelman AB, Gallo MF, Jensen JT, Nichols MD, Schul...»1 included 12 studies with a total of 3 628 subjects.
Study findings for oral contraceptives were similar between 28-day and extended cycles in regard to contraceptive efficacy (i.e., pregnancy rates) and safety profiles. When compliance was reported, no difference between 28-day and extended cycles was found. Participants reported high satisfaction with both dosing regimens.
The few studies that reported menstrual symptoms found that the extended cycle group fared better in terms of headaches, genital irritation, tiredness, bloating, and menstrual pain. However, breast tenderness was increased in continuous dosing of the contraceptive patch. Eleven out of the twelve studies found that bleeding patterns were either equivalent between groups or improved with extended or continuous cycles. Endometrial lining assessments by ultrasound were done in a small number of participants and were all normal
The studies were too small to address efficacy, rare adverse events, and safety.
Comment: The quality of evidence is downgraded by imprecise results (limited study size for each comparison).