The minipill, also known as the progestin-only birth control pill, is an oral contraceptive that contains the hormone progestin. Unlike combination birth control pills, the minipill doesn't contain estrogen. The progestin dose in a minipill is lower than the progestin dose in a combination oral contraceptive pill.
The minipill thickens cervical mucus and thins the lining of the uterus (endometrium) — preventing sperm from reaching the egg. The minipill also sometimes suppresses ovulation. For maximum effectiveness, you must take the minipill at the same time every day.