The minipill norethindrone is an oral contraceptive that contains the hormone progestin. Unlike combination birth control pills, the minipill — also known as the progestin-only pill — doesn't contain estrogen. The progestin dose in a minipill is lower than the progestin dose in a combination birth control pill.

The minipill thickens cervical mucus and thins the lining of the uterus (endometrium) — preventing sperm from reaching the egg. The minipill also suppresses ovulation, but not consistently. For maximum effectiveness, you must take the minipill at the same time every day.

Your health care provider may recommend the minipill if:

• You're breast-feeding. For years it was thought that the estrogen in combination birth control pills inhibited breast milk production. Although recent research has found that combination birth control pills don't affect lactation, many providers and patients still have experience and confidence in the minipill for breast-feeding.
• You have certain health problems. If you have a history of blood clots in the legs or the lungs, or if you have an increased risk of those conditions, your doctor might recommend the minipill.
• You're concerned about taking estrogen. Some women choose the minipill because of possible side effects of birth control pills containing estrogen.

The minipill is an easily reversible method of contraception. Your fertility is likely to return to normal immediately after you stop taking the minipill.

Your health care provider might also recommend the minipill to help treat a type of skin inflammation (dermatitis) that seems to be related to your menstrual cycle.

The minipill isn't appropriate for everyone, however. Your health care provider may discourage use of the minipill if:

• You have or have had breast cancer
• You have liver disease
• You have unexplained uterine bleeding
• You're taking medications for tuberculosis, HIV/AIDS or to control seizures
• You'll have trouble taking the pill at the same time every day due to a changing work schedule or other factors

It's estimated that as many as 13 out of 100 women who use the minipill will get pregnant in a year of use. The failure rate of the minipill is thought to be higher than that of other hormonal contraceptive methods. In addition, the minipill won't protect you from sexually transmitted infections.

If you become pregnant while taking the minipill, there appears to be a slightly higher chance that the fertilized egg will implant outside the uterus, usually in a fallopian tube (ectopic pregnancy). However, there doesn't appear to be an increased risk of birth defects in babies born to women taking the minipill while pregnant.

Side effects of the minipill might include:

• Irregular menstrual bleeding
• Acne
• Breast tenderness
• Decreased sex drive (libido)
• Depression
• Headaches
• Nausea
• Ovarian cysts

You'll need a prescription for the minipill from your health care provider.

As long as you aren't pregnant, you can start taking the minipill anytime — ideally on the first day of your menstrual period. Your health care provider might recommend using a backup method of birth control, such as a condom, for the first two days after you start taking the minipill.

You might be able to skip the backup birth control if you start taking the minipill:

• During the first five days of your period
• Between six weeks and six months after giving birth if you are fully or nearly fully breast-feeding and haven't had a period
• Within the first 21 days after giving birth if you're not breast-feeding
• The day after you stop using another hormonal method of contraception
• Immediately after pregnancy termination

If you're switching from a combination birth control pill to the minipill, start taking the minipill the day after you take your last active combination birth control pill.

To use the minipill:

• Consult your health care provider about a starting date. Make sure you have a backup method of birth control available if necessary.
• Pick a time to regularly take the pill. It's important to take the minipill at the same time every day. If you take the minipill more than three hours later than usual, avoid sex or use a backup method of birth control for at least two days.
• Be cautious with missed pills. If you miss a minipill, take the missed pill as soon as you remember — even if it means taking two pills in one day. Use a backup method of birth control for at least two days. If you've had unprotected sex, consult your health care provider about emergency contraception.
• Don't take breaks between packs. Always have your next pack ready before you finish your current pack. Unlike combination birth control pills, minipill packs don't contain a week of inactive pills.

If you're taking antibiotics or you experience vomiting or diarrhea while using the minipill, use a backup method of birth control. If your bleeding is particularly heavy or lasts for more than eight days, consult your health care provider.