Mad Monkey, on Aug 30 2009, 15.14, said:
That's hardly the same thing. Tansy is something that really exists. Comparing it to something that for our purposes only exists within the ASOIAF world doesn't really work. It's like saying we shouldn't assume that "swords" in ASOIAF are the sharp, pointy weapons they are in real life. Cersei believes a lot of things that are not true, and while I agree with you that she is right about the tansy tea, her word alone in this matter isn't enough to convince me. After all, she also believes that Kevan is plotting with the Tyrells, that she herself is a great queen, and that things that you force someone to say represent their true feelings. The other references to Tansy tea, such as with Lady Lysa and Hoster Tully, are more plausible than Cersei's fantasies.
Martin has countless of examples in the books where he takes things that actually exist and reformats them to have the properties that he needs to tell his stories. By your logic:
1) Winter exists in Westeros
2) Winter exists on earth
3) Winters on earth exist for roughly 3 months
4) Therefore, winters on Westeros exist for rougly 3 months
Genetics is another good example. Martin takes a basic concept (children tend to inherit traits of their parents, with some parents' traits being dominant), and blasts them off into outer space with family trees and character traits that simply cannot be explained in a Mendelian model.
More to the point, even if tansy in westeros == tansy on earth, it does not necessarily follow that the Westerosi denizens would know of any of its additional properties or would use it for anything other than an abortifacient.
And, as has been pointed out, tansy is but one of apparently many ingredients in Moon Tea. Lysa lists mint, wormwood, honey, and pennyroyal as other ingredients.