Combined oral contraceptive pill
Role of placebo pills

The role of the placebo pills is two-fold: to allow the user to continue the routine of taking a pill every day and to simulate the average menstrual cycle. By continuing to take a pill every day, users remain in the daily habit even during the week without hormones. Failure to take pills during the placebo week does not impact the effectiveness of the pill, provided that daily ingestion of active pills is resumed at the end of the week.

The placebo, or hormone-free, week in the 28-day pill package simulates an average menstrual cycle, though the hormonal events during a pill cycle are significantly different from those of a normal ovulatory menstrual cycle. Because the pill suppresses ovulation (to be discussed more in the Mechanism of action section), birth control users do not have true menstrual periods. Instead, it is the lack of hormones for a week that causes a withdrawal bleed. The withdrawal bleeding that occurs during the break from active pills has been thought to be reassuring, a physical confirmation of not being pregnant. The withdrawal bleeding is also predictable. Unexpected breakthrough bleeding can be a possible side effect of longer term active regimens.

Since it is not uncommon for menstruating women to become anemic, some placebo pills may contain an iron supplement. This replenishes iron stores that may become depleted during menstruation. As well, birth control pills, such as COCPs, are sometimes fortified with folic acid as it is recommended to take folic acid supplementation in the months prior to pregnancy to decrease the likelihood of neural tube defect in infants.
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