Well, ok, not RU 486, but those morning after pills (which are different, if I’m not mistaken). How did they do the studies to determine if they work? Most instances of unprotected sex do not result in pregnancy. The drugs have to be used with 72 hours–so how did the researchers know the subjects were in fact pregnant, and that the pregnancies were, uh, avoided by the drugs?
What did they do, gather 2000 women who had unprotected sex the night before, give 1000 a placebo and 1000 the drug, and find 3 with the placebo got pregnant and only one with the drug?
The “morning-after pill” is just ordinary oral contraceptives, in a higher-than-normal dose. It prevents the zygote from attaching to the uterus wall and is about 75% effective (compared to no contraceptive). Exactly how they determined that, I don’t know. See http://www.sexhealth.org/infocenter/GuideSS/morning.htm
There is evidence that RU486 can be used as a morning-after pill as well, but it is primarily intended to induce abortion at a later stage.