BIG WORD ALERT!!!!!!! :scared: (Added by IdahoHerbalist)
Like most plants, yarrow has numerous chemicals that can impact human physiology. There are a lot of anti-oxidants and flavanoids that can help lower blood pressure. It also has some ACE inhibiting capacity. Yarrow also causes peripheral vasodilation which will decrease overall blood pressure.
So, how is yarrow a vasodilator and a vasoconstrictor? In addition to the blood-pressure-lowering constituents above, yarrow also contains a lot of tannins. So, when used topically, the tannins cause astringency and vasoconstriction locally. They may be doing so to some extent internally as well…I don’t know. Bear in mind too that decreasing blood pressure also decreases bleeding. Taking yarrow internally has an effect similar to raising the bleeding limb above the head to slow the bleeding. :0)
Yarrow also contains an alkaloid called achillene that decreases clotting times and stops bleeding in ways similar to shepherd’s purse by supporting clot formation (mostly thorugh it’s effects on fibrin).
Yarrow also increases uterine contraction. So, that property combined with the properties above make it quite good for post-partum hemorrhage.
Did I mention that it also has a bazillion other properties that make it wildly useful for all kinds of other stuff? :0)
Doc
Don't use herbs or combine herbs with medications or use them during lactation or pregnancy without talking with your healthcare provider.