With increasing over-the-counter availability of "Plan B" emergency contraception, many people are wondering if Plan B can be a way out of an unwanted pregnancy.
The answer is: No, plan B will not cause a miscarriage or induce an abortion in an existing pregnancy. Although there are prescription-only drugs that can terminate a pregnancy, plan B does not fall into that category.
Instead, Plan B falls in the category of emergency contraception. When taken within a few days of unprotected sex, before a woman is technically pregnant yet, emergency contraception will reduce the odds of pregnancy occurring. Because one possibility is that plan B may impede implantation of a fertilized egg, people who strongly believe in pregnancy beginning at fertilization may claim that plan B will cause an abortion, but after implantation has already taken place (which occurs roughly 8 to 12 days after conception), plan B will not cause an abortion or induce a miscarriage.
By the time it is possible to confirm that you are pregnant (as in get a positive result on a home pregnancy test), implantation has already taken place and taking plan B will not have any effect. If you are currently dealing with an unplanned pregnancy, you will need to look into other resources.
Plan B Emergency Contraception. McKinley Health Center. Accessed: Mar. 31, 2009. http://www.mckinley.uiuc.edu/Handouts/plan_b_contraception.html.