After I summarized how Plan B contraception works, I’m still getting letters confusing it with RU486. RU486 induces abortions. Plan B does not. RU486 is the opposite of Plan B.

Remember that what Plan B is is an artificially high dose of progesterone (it actually uses a progesterone analog, but it’s effectively the same.) Progesterone is a hormone that maintains the uterine lining in a nice, rich, spongy, receptive state, and it also suppresses another hormone, LH, that is what triggers ovulation. Plan B keeps the uterus primed for implantation, but tells the ovary to hold its fire and not release an egg.

RU486 can’t get much different. It’s a compound, mifepristone, that antagonizes progesterone—it binds to progesterone receptors and blocks their function, so that it looks to the cells as if progesterone levels have all all dropped to zero. The cells of the uterus, whether implantation has occurred or not, are tricked into menstruating right away, shedding the uterine lining and anything growing in it.

Now I personally think RU486 is a fine idea and a perfectly reasonable and relatively safe way to induce an abortion, and I think it ought to be legal and available. However, it is nothing like Plan B. Plan B is a completely separate issue from any argument over the ethics or utility of abortion.