Yarrow herb, like many plants and herbs, contains many flavonoids including apigenin and luteolin. Different species from yarrow are used against gastrointestinal and hepato-biliary disorders in traditional European medicine. Very little human research is available on the medicinal uses of yarrow herb. In lab studies, flavonoids from yarrow have been shown to help relieve smooth muscle spasms and to promote bile acid flow.

Human studies
I could not find published studies with yarrow herb as of 2016.

Animal studies, spasmolytic effect
In isolated guinea pig ilea, flavonoids from yarrow -- the aglycones querctin, luteolin and apigenin -- exhibited high antispasmodic activities mainly by blockade of the calcium inward current, but additionally also by mediator-antagonistic effects. Spasmolytic or antispasmodic refer to a substance used to relieve or prevent spasms (especially of the smooth muscles).

Composition of yarrow herb
Scientists have isolated flavonoids such as apigenin, luteolin, centaureidin, casticin and artemetin, along with sesquiterpenoids including paulitin, isopaulitin, psilostachyin C, desacetylmatricarin and sintenin.

Clinical uses
The genus Achillea consists of about 140 perennial herbs native to the Northern hemisphere. Traditional uses include digestive problems, liver and gall-bladder conditions, menstrual irregularities, cramps, fever, and wound healing. Externally it is used in the form of sitz bath or as a compress against skin inflammation, slow healing wounds, bacterial or fungal infections.

Evid Based Complement Alternat Med. 2013. Antimutagenic Effect of Medicinal Plants Achillea millefolium and Bauhinia forficata In Vivo.

Traditional uses
Yarrow has been used traditionally for its astringent effects. Yarrow decoctions have been used to treat inflammations such as piles (hemorrhoids), and also as a treatment for headaches.

J Membr Biol. 2013. Minireview on Achillea millefolium Linn. Akram M. Yarrow (Achillea millefolium) is an important medicinal plant with different pharmaceutical uses. A. millefolium has been used for centuries to treat various diseases including malaria, hepatitis and jaundice. A. millefolium is commonly prescribed to treat liver disorders. It is also used as an anti-inflammatory agent and is a hepatoprotective herb. A. millefolium is considered safe for supplemental use. It has antihepatotoxic effects also. It is prescribed as an astringent agent. It is prescribed in hemorrhoids, headache, bleeding disorders, bruises, cough, influenza, pneumonia, kidney stones, high blood pressure, menstrual disorders, fever, rheumatoid arthritis, gout, osteoarthritis, hemorrhagic disorders, chicken pox, cystitis, diabetes mellitus, indigestion, dyspepsia, eczema, psoriasis and boils.

Yarrow herb studies
Choleretic effects of yarrow (Achillea millefolium) in the isolated perfused rat liver.
Phytomedicine. 2005;
Department of Pharmacognosy, University of Vienna, PharmaCenter Vienna, Althanstrasse Vienna, Austria.
In this work, a fraction enriched in dicaffeoylquinic acids and luteolin-7-O-beta-d-glucuronide was investigated on its choleretic effect in the isolated perfused rat liver compared to cynarin, the main choleretic compound of Cynara scolymus L. A fraction containing 3,4-, 3,5- and 4,5-dicaffeoylquinic acid and luteolin-7-O-beta-d-glucuronide was prepared by solid phase extraction from a 20% methanolic extract of yarrow. A total amount of 48% dicaffeoylquinic acids and 3% luteolin-7-O-beta-d-glucuronide was determined with cynarin as internal standard. Experiments revealed a dose-dependant increase in bile flow (23-44-47%) by the Achillea fraction. Choleresis was two- to three-fold higher than that of cynarin. The combined effect of dicaffeoylquinic acids and luteolin-7-O-beta-d-glucuronide stimulated bile flow more effectively than the single compound cynarin. Due to their polar structure, these compounds are quantitatively extracted into teas and tinctures; hence, they seem to be the choleretic active principles in the traditional application forms of yarrow.

Experimental study of the effect of the phytomixture made of leaves of Plantago major L. and Achillea millenfolium L. - yarrow - on the secretion activity of the stomach in dogs
Eksp Klin Gastroenterol. 2005.
In this study, the author used plants with an obvious anti-ulcerous effect, which are often jointly included in gastroenterological phytomixtures. It was shown that the extract from leaves of Plantago major L. has a stimulating effect on gastric secretion, mostly on parietal cells. The extract from yarrow reduced aggressive properties and enhanced protective properties of gastric juice. The common effect was demonstrated by the increased acid production and enhanced protective properties of gastric mucus.

Chemical Variability in the Essential Oil Components of Achillea millefolium Agg. ( yarrow ) from Different Himalayan Habitats (India).
Planta Med. 2005.
Regional Research Laboratory, Jammu (J and K), India.
Essential oil diversity was studied in wild Achillea millefolium (yarrow) from two different high altitude Himalayan habitats (1600 m, 2850 m) and their cultivated populations under uniform environmental conditions at lower altitudes of Jammu (300 m). The populations proved to represent two different ecotypes: the 1,8-cineole type and the borneol type with appreciable differences in the contents of oils and mono- and sesquiterpenes. Populations from all these habitats showed considerable overlap in various constituents and the major components were characterized as beta-pinene (10 % - 17 %), 1,8-cineole (3 % - 15 %), borneol (0.2 % - 12 %), and beta-caryophyllene (8 % - 16 %). No variation in morphology and chromosome number was observed under comparable environmental conditions from different habitats. Preliminary investigation indicates the existence of different ecotypes from the Himalayan habitats.

Yellow dock used for treating animals
Achillea millifolium, commonly called 'Pehl-ghasa' in India, is used by farmers in traditional animal health care as a vermifuge. the entire plant of A. millifolium possesses significant anthelmintic activity and could be a potential alternative for treating cases of helminth infections in ruminants.