The idea of administering small daily amounts of a progestin arose when clinical experience with some of the low-dose combination pills indicated that contraception was being provided even though ovulation was not always inhibited. Subsequent studies demonstrated that a small daily quantity of a progestin alone would provide reasonably good protection against pregnancy without suppressing ovulation. The method has the following advantages: (1) Because no estrogen is given, the side effects attributable to the estrogen component of conventional oral contraceptives are eliminated; (2) the minipill is taken every day; thus no special sequence of pill-taking is necessary. The mechanism of contraceptive action of the microdose nonstop progestins is not known. It has been suggested that the cervical mucus becomes less permeable to sperm and that endometrial activity goes out of phase, so that nidation is thwarted even if fertilization does occur. In clinical tests, the use of microdoses of progestins has resulted in a pregnancy rate of about 2-7 per 100 woman years.

Progestin is associated with some side effects, mainly irregularity of the ovulatory cycle and ectopic pregnancies, and these significantly reduce its contraceptive acceptability. Its overall effectiveness is less than that of combination pills. Currently, the minipill is thought to be useful in only a few patients, ie, in those having a documented hypersensitivity to estrogens and perhaps for the lactating woman.

Postcoital or Emergency contraception
Postcoital or emergency contraception is a therapy used to prevent unwanted pregnancy after unprotected intercourse or after a failure of a barrier method. There are various methods to achieve emergency contraception. The Yuzpe method is the most widely prescribed. It consists of two tablets, each containing ethinyl estradiol 0.05 mg and 0.5 mg norgestrel ingested 12 hours apart for a total of 4 tablets. When the therapy is initiated within 72 hours of intercourse the effectiveness is about 74%. While it is recommended to start therapy within 72 hours, there have been some studies that showed protection up to 120 hours later. Nausea occurs in 30-60% of patients who receive emergency contraception. Emesis occurs in up to 22% of patients. Some prescribers administer antiemetics 1 hour before the pills are taken. Progestin-only post-coital contraception is also available and possibly better tolerated due to less nausea with comparable success rates to combined methods. Levonorgestrel at a dose of .750 micrograms is given by mouth within 72 hours of intercourse and repeated 12 hours later. Patients are also advised to seek medical attention if menses have not begun within 21 days after treatment.