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The Natural Medicines Comprehensive Database has no data on the effectiveness of yarrow. Sounds like an opportunity for a study. (A bit out of my scope of practice, though.)
/*serious*
Yarrow (Achillea Millefolium)
Effects: The herb has a cholagogue effect (stimluates bile flow) due to the guaianolide and germacranolide content. The flavenoid content exerts a spasmolytic efffect, while the proazulene fraction has an anti-edema and anti-inflammatory effect. The effect probably results from the interaction of the various structured bonds with the chamazulene and flavenoids. Yarrow has similar effects to those observed in chamomile flowers since some of their components are identical.
Note: the sespuiterpene lactone, alpha-peroxyachfolid may cause skin dermititus.
Compounds: volatile oil (.2-1%) chamazulene, camphor, beta-pinene