Plan B consists of two relatively large doses of a single ingredient, levonorgestrol, a constituent of many birth control pills.
In the course of their deliberations, the committees voted twenty-eight to zero that the drug was safe (one member of NDAC commented that the single ingredient of Plan B, levonorgestrol, is the safest drug the committee had yet considered); they voted twenty-seven to one that consumers could properly use Plan B as recommended on the proposed labeling (as judged from the "actual use study" that was part of the sponsor's application); they voted twenty-eight to zero that women were unlikely to use Plan B as a regular form of contraception; and they voted twenty-seven to one that the actual use study data were generalizable to the overall population of over-the-counter users, including adolescents.
Second, because the mechanism by which the drug prevents pregnancy isn't definitely known, some on the committees argued that levonorgestrol could, at least at times, prevent implantation of a fertilized ovum, which some view as a form of abortion, hence unacceptable.
The available evidence on mechanism of action, limited as it is, strongly indicates that levonorgestrol is a contraceptive, rather than an abortifacient; that is, it appears to prevent fertilization rather than preventing implantation of the fertilized ovum.
It consists of six tiny rods containing the progestin hormone levonorgestrol. These are implanted under the skin of a woman's inside arm.
Vaginal rings containing levonorgestrol, which can be inserted and removed by the woman, are also being tested.