Details:

Levonorgestrel is a hormonal medication which is used in a number of birth control methods. It is combined with an estrogen to make combination birth control pills. As an emergency birth control, sold under the brand name Plan B among others, it is useful within 72 hours of unprotected sex. The more time that has passed since sex, the less effective the medication becomes, and it does not work after pregnancy (implantation) has occurred.] It decreases the chances of pregnancy by 57 to 93%.In an intrauterine device (IUD), such as Mirena among others, it is effective for the long-term prevention of pregnancy. A levonorgestrel-releasing implant is also available in some countries.

Common side effects include nausea, breast tenderness, headaches, and increased, decreased, or irregular menstrual bleeding. When used as an emergency contraceptive, if pregnancy occurs, there is no evidence that its use harms the fetus. It is safe to use during breastfeeding .Birth control that contains levonorgestrel will not change the risk of sexually transmitted infections. It is a progestin and has effects similar to those of the hormone progesterone. It works primarily by preventing ovulation and closing off the cervix to prevent the passage of sperm.

Use
Levonorgestrel comes as a tablet to take by mouth. If you are taking levonorgestrel as a single tablet product , take one tablet as soon as possible within 72 hours after unprotected sexual intercourse. If you are taking levonorgestrel as a two tablet product, take one tablet as soon as possible within 72 hours after unprotected sexual intercourse and take a second dose 12 hours later. Levonorgestrel works best if it is taken as soon as possible after unprotected sexual intercourse. Follow the directions on your prescription label carefully, and ask your doctor or pharmacist to explain any part you do not understand. Take levonorgestrel exactly as directed.

If you vomit less than 2 hours after you take a dose of levonorgestrel, call your doctor. You may need to take another dose of this medication.

Because you can become pregnant soon after treatment with levonorgestrel, you should continue using your regular method of birth control or begin using regular birth control immediately.

Side Effects
heavier or lighter than usual menstrual bleeding
spotting or bleeding between menstrual periods
nausea
vomiting
diarrhea
tiredness
headache
dizziness
breast pain or tenderness