Within 72 hours of unprotected intercourse, a birth control pill can be taken to reduce the likelihood of pregnancy resulting from this exposure.

A single dose of levonorgestrel 150 mcg, taken within 3 days of exposure, will reduce the chance of resulting pregnancy from about 8% to about 1%. The trade name for this product is the Plan B One-Step.

This dosage is accompanied by a 14% risk of significant nausea, so it’s not a good choice for routine contraceptive use. But for emergency use, it is pretty effective.

Following use of this medication, the next menstrual period may come on time, or be a little early or a little late. 95% of women using this medication will have their next periods within 7 days of the expected time. The next period may be of average heaviness and duration, or it may be heavier or lighter. If the next period is more than 7 days late, a pregnancy test should be performed.

With progestin-only contraception taken continuously, there is a somewhat increased risk of tubal ectopic pregnancy when the contraception fails. No one knows whether that same risk applies to this single dose of levonorgestrel or not. But to be safe, should there be an emergency contraception failure, it’s a good idea to rule out ectopic pregnancy.

In the past, there was some concern that emergency contraception might cause abortions rather than simply prevent pregnancies, but those concerns have since been proven to be unfounded with levonorgestrel. It prevents pregnancy…it does not cause abortions. In the event the patient is already pregnant when she takes this medication, it will not cause her to lose the pregnancy and will also have no effect on the developing fetus. It is believed to work by interfering with ovulation and alterations in sperm transport.

While the single, 150 mcg levonorestrel, is probably the best choice for emergency contraception, there are some other good alternatives.

Two Ovral birth control pills can be taken, followed 12 hours later by two more Ovral pills. Should Ovral not be available, other good alternatives include any levonorgestrel-containing birth control pill, including:

• Lo-Ovral
• Nordette
• Levlen
• Levora
• Portia

If any of these pills are used, the dosing is 4 pills immediately, followed by 4 more pills 12 hours later. This dosage will reduce the risk of pregnancy from 8% to about 2%. So, it is somewhat less effective than the single 150 mcg dose of levonorgestrel, and also has a higher risk of nausea, approaching 50%.

The greatest experience with emergency contraception has been within the 72-hour window between exposure and taking the pill. Some studies find emergency contraception is most effective the sooner it is initiated within that 72 hours. Other studies find no difference in pregnancy rates. A few studies have looked at the use of emergency contraception for up to 120 hours after unprotected intercourse and find that it can still be effective in some cases, even after 72 hours.

Contraindications to the use of birth control pill based emergency contraception are essentially the same as those for use of the birth control pill in general, and are due to its estrogen content. Previous stroke, undiagnosed uterine bleeding, heart attack, deep vein thrombophlebitis and cancer of the breast or uterus are all contraindications to sustained pill use. The extent to which they represent risks in the context of emergency contraception is not known, but should be weighed in evaluating a patient for emergency contraceptive use.

However, for the single 150 mcg levonorgestrel dose, which contains no estrogen, there are no contraindications, other than known or suspected pregnancy.