MOON TEA - a decoction of herbs of a certain composition, a popular means for terminating pregnancy in Westeros. It is drunk by women who are already pregnant or who do not want to get pregnant. The recipe for making moon tea is not a secret: it is known both by the Maesters of the Citadel, as well as village healers and even wildling forest witches. Tansy and Marshmint are two natural, powerful (and DANGEROUS) abortions. They can lead to death if used unthinkingly.
The only contraceptive that is mentioned in the book (but apparently not in the series) is "moon tea". A decoction of herbs that can prevent or even terminate unwanted pregnancies.