The Food and Drug Administration (FDA) recently approved a new emergency contraceptive that's available by prescription. Ulipristal acetate (ella), a selective progesterone receptor antagonist and partial agonist that prevents progesterone from occupying its receptor, must be taken within five days (120 hours) after a suspected contraceptive failure or unprotected intercourse.

Although the drug can be taken at any time in the menstrual cycle, its mechanism of action varies depending on when it's administered. The most probable explanation for the drug's effectiveness is its ability to inhibit or delay ovulation. If it's administered during the midfollicular phase, it inhibits folliculogenesis and reduces the estradiol concentration. If it's administered during the time of the luteinizing hormone peak, it will delay follicular rupture for five to nine days. Dosing in the early luteal phase doesn't significantly delay endometrial maturation, but it does decrease endometrial thickness. The effect on the endometrium may affect implantation.

The drug shouldn't be administered if the patient is already pregnant; it cannot be used to induce abortion. Its effect on a fetus is unknown. Ulipristal has been available by prescription in Europe since May 2009 under the trade name ellaOne. The adverse effects of ulipristal are similar to those of other levonorgestrel-based emergency contraceptives, such as Plan B and Next Choice, and include headache, nausea, abdominal pain, pain or discomfort during menstruation (dysmenorrhea), fatigue, and dizziness. Patients should be assessed to ensure that they're not pregnant before they're given a prescription for ulipristal. To read the drug's full prescribing information, go to http://bit.ly/aAbrzQ.