"Yarrow has a high reputation and is widely employed in herbal medicine, administered both internally and externally. It is used in the treatment of a very wide range of disorders but is particularly valuable for treating wounds, stopping the flow of blood, treating colds, fevers, kidney diseases, menstrual pain etc[218, 238, 254, 257]. The whole plant is used, both fresh and dried, and is best harvested when in flower. Some caution should be exercised in the use of this herb since large or frequent doses taken over a long period may be potentially harmful, causing allergic rashes and making the skin more sensitive to sunlight. The herb combines well with Sambucus nigra flowers (Elder) and Mentha x piperita vulgaris (Peppermint) for treating colds and influenza. The herb is antiseptic, antispasmodic, mildly aromatic, astringent, carminative, cholagogue, diaphoretic, digestive, emmenagogue, odontalgic, stimulant, bitter tonic, vasodilator and vulnerary[4, 9, 14, 21, 54, 145, 165, 172, 238, 240, 254]. It also contains the anti-inflammatory agent azulene, though the content of this varies even between plants in the same habitat. The herb is harvested in the summer when in flower and can be dried for later use"
- PFAF References