While yarrow is often used by humans for its many medicinal benefits, the toxins within the plant do provide a risk of potential poisoning if a cat were to eat copious amounts of it. This is unlikely, as yarrow plants contain hydrosoluble tannins that are very bitter to the taste, preventing most cats from eating more than one bite. The achilleine glycoalkaloids, sesquiterpene lactones, and monoterpenes found in the plant can cause a general upset to the gastrointestinal system. There is also a risk of increased complications in cats who have many allergies, are pregnant or nursing, or who acquire wounds or incisions after yarrow consumption. Sesquiterpene lactones may also cause adverse skin reactions on the cat. 

The yarrow is a very common perennial herb that can be found throughout North America. It is a slender plant, with green, feathered leaves tapering up the stem. Blossom clusters are found at the very top of the plant, and come in various colors. White or pale yellow are the most common colors of yarrow found in the wild. Hybrid yarrow may be used in gardens and can be bright yellow or red in color. It blooms all summer long, right into the fall. Yarrow goes by multiple names including milfoil, soldier's woundwort, bloodwort or field hops, but its scientific name is actually Achillea millefolium. It is a part of the Asteraceae family of plants. 

Unless other issues exist within the cat, gastrointestinal distress is the most common response after too much plant material has been ingested. Signs to watch for are listed as follows:

Yarrow can be found growing naturally on roadsides and in forests or grasslands. Hybrid varieties are often used in landscaping and can be found in many gardens. Any cat who has outdoor exposure may come into contact with yarrow. All parts of the plant that grow above the ground contain toxins. Bitter-tasting tannins prevent most animals from ingesting a harmful amount of yarrow.

For a cat to be poisoned from eating yarrow, extremely large portions will need to have been eaten. This means that it is likely that you will either witness the cat eating the plant, or you will see what is obviously plant material in your cat's vomit or excrement. Be sure to let your veterinarian know of this, and even bring a small sample if possible, so that the plant can be identified if you are unsure of what it is. 

You will be asked to provide your cat's full medical history to the veterinarian, to help rule out other possible causes of gastrointestinal sickness. You may be asked if your cat is allowed outdoors, and if you live near a rural area. A complete physical examination may be performed by the veterinarian to note all present symptoms. Full blood work may be run to assess the overall health of the cat and pinpoint a poisoning as the only possible cause of symptoms by process of elimination.

Treating a cat who has been poisoned by Yarrow will mainly have to do with managing symptoms. Most yarrow poisonings are relatively mild, however, in some instances, hospitalization may be necessary.

If the cat has been expelling its gastrointestinal contents for a long period of time, it is not unusual for it to become dehydrated. Intravenous fluid administration can help rehydrate the cat to keep it in the best possible condition while dealing with the effects of the poison.

As most yarrow poisonings require a large amount of the plant to be ingested, it is likely that giving the cat activated charcoal will help, as all remaining toxins will bind together and pass through the intestines undigested. 

Consuming even large amounts of yarrow is generally not life-threatening to cats. There are actually some remedies made for cats that contain very small portions of the plant (mainly for topical therapies). Most cats will make a full recovery once all plant material has passed through the body. No lasting damage should occur from the poisoning.

To prevent your cat from coming into contact with Yarrow, it may be best to keep it indoors where you can control which plants are it is exposed to. If your cat has eaten yarrow, it may be necessary to postpone any surgeries that may have been scheduled for the cat, as yarrow may interfere with the clotting process. If a mother cat has consumed yarrow, prevent it from nursing its kittens and instead give them kitten formula.