The morning after pill shoots a massive amount of hormones through the body to stop ovulation immediately. This is not healthy and should only be done if the first method fails.
This pill is not the same as RU-486. It is not a poison and will not harm a conceived embryo. It stops ovulation and thickens the mucus to stop sperm.
Wouldn’t it be reasonable to assume eventual internal damage from taking such a hormone blaster once each month for 5 years?
Actually, one way that “plan b” works is to prevent implanation of the embryo, since conception precedes implantation by about 7-10 days. If “pregnancy” is defined as beginning at implantation, then yes, plan b is unlikely to harm an established pregnancy. From the FDA website:
“Plan B works like other birth control pills to prevent pregnancy. Plan B acts primarily by stopping the release of an egg from the ovary (ovulation). It may prevent the union of sperm and egg (fertilization). If fertilization does occur, Plan B may prevent a fertilized egg from attaching to the womb (implantation). If a fertilized egg is implanted prior to taking Plan B, Plan B will not work.”
http://www.fda.gov/CDER/drug/infopage/planB/planBQandA.htm
BTW, preventing implanation is also one of the ways that the IUD works, as well as ordinary birth control pills, in some cases.