What Are Progestin-Only Pills?

Progestin-only pills are birth control pills that contain only progestin, the synthetic version of the hormone progesterone, instead of containing estrogen and progesterone, like combination pills do. The pills that contain only progestin are often referred to as mini-pills because of their lower levels of hormones than regular oral contraceptives. Progestin-only pills prevent ovulation in about half of the women who take them. They also help prevent pregnancy by thickening the cervical mucus, making it more difficult for sperm to reach an egg, and by thinning the lining of the uterus, making it less likely that an egg would implant if fertilized.

Women who are breastfeeding often take progestin-only pills to prevent pregnancy. The low dose of progestin and the absence of estrogen make the pills safe for breast milk. Progestin-only pills also are safer for women who are older than age 35, especially if they smoke. Combination pills can lead to serious complications, including blood clots, in older women who smoke, so the mini-pills are safer. Other women who might use progestin-only pills include those with diabetes, high blood pressure, heavy menstrual periods or sickle cell disease, as well as women who want an easily reversible form of birth control, because fertility usually returns very quickly after stopping the use of progestin-only pills.

Pills that contain only low doses of progestin have a higher failure rate than combination pills. With perfect use, progestin-only pills are about 99.5 percent effective, which is only slightly less than the rate for combination pills. Mini-pills must be taken at the exact same time every day to constitute perfect use, however. Women should strive to take any birth control pill at the same time each day, but it is particularly important with mini-pills because they can decrease in effectiveness drastically by taking them even a few hours late. Progestin-only pills are 92-95 percent effective with typical use.

Forgetting to take a progestin-only pill increases the risk of pregnancy much more than missing a single combination pill. Women who take their mini-pill more than three hours after the usual time should use a backup method of birth control for at least 48 hours. If a pill is completely missed, the woman must use a backup method until her next menstrual period to decrease the risk of pregnancy. Unlike combination pills, women who take mini-pills cannot take two the next day to make up for a missed pill.

The side effects associated with progestin-only pills are similar to the side effects of combination pills. The most common side effects include acne, headache, nausea, breast tenderness and weight changes. Some women also experience irregular menstrual periods, depression or ovarian cysts. Most of the side effects are minor, and many of them resolve themselves after several weeks or months of taking the pills.