Despite its possible medical applications, yarrow probably isn't something you want to give to your cat. In a pinch, it might help treat internal or external wounds, but it could also make your cat vomit or get sick.
Yarrow (Achillea millefolium) is a plant with a variety of applications for humans and animals, in traditional and modern herbal medicine. It's known by a number of common names, including milfoil, and grows in temperate parts of the Northern Hemisphere. Yarrow contains chemicals that can affect blood pressure and can have anti-inflammatory effects, according to WebMD. The leaves and flowers are harvested for medication. North Carolina State University Cooperative Extension and the American Society for the Prevention of Cruelty to Animals list these same plant parts as poisonous to humans and animals.
In humans, yarrow treats fever, the common cold, hay fever, dysentery, diarrhea, loss of appetite and gastrointestinal tract discomfort. It also induces sweating. It contains an effective anticoagulant, which accounts for its topical use on external wounds. Some herbalists, including certified master Herbalist Sue Sierralupe, advocate its use to treat similar conditions in cats and other animals. This is particularly true of of topical wounds, which cats are wont to lick. In "The Natural Remedy Book for Dogs and Cats," Diane Stein recommends using yarrow to treat fever, infectious diseases, and internal and external bleeding, as well as to cleanse the liver, kidneys and bladder.
Yarrow is toxic to cats, dogs and horses, according to the American Society for the Prevention of Cruelty to Animals. Technically, the principal toxins appear to be glycoalkaloids, such as achilleine and moschatine, hydrolysable tannins, monoterpenes, sesquiterpene lactones and colatile oils. Yarrow ingestion may cause vomiting, diarrhea, depression, increased urination, anorexia and hyper-salivation. The American Society for the Prevention of Cruelty to Animals says to contact poison control if your pet ingests this plant, although the wording of the warning is identical to those that appear elsewhere on the site -- it's not a yarrow-specific warning. Yarrow poisoning is rare, according to a Paws Dog Day Care article, largely because its bitter taste "tends to dissuade animals from overconsumption."
Given the ambiguity surrounding it, you should err on the side of caution and avoid letting your cat eat yarrow or treating him with any herbal medication that include it. If your cat shows classic poisoning symptoms -- vomiting, diarrhea, weakness and the like -- call a veterinarian or pet poison control hotline. It can take several hours for poisoning symptoms to manifest, so timing is of the essence. You can figure out if yarrow is the poisoning culprit later. You need to make sure cat is OK first. Unless you're in a field and yarrow is the only thing available to stop your cat from bleeding out, you're not under pressure to use it; plenty of alternatives, including nontoxic, laboratory-tested herbs and medicines, that can help your cat.