Dani and her horse Polo
Phenylbutazone, commonly referred to as “bute”, is a medication similar to aspirin that is aimed primarily at horses, cows, and other large mammals for management of pain.  It also helps reduce fever and inflammation.  Originally designed for humans suffering from arthritis pain, it was found to cause unpleasant side effects, such as vomiting and diarrhea. [1,2] It may even have more potentially life-threatening consequences. There is a great risk of increasing ulcers or bleeding in your stomach or intestines, it can cause painful swelling from fluid backup in your limbs and lungs, and has been related to a type of blood disorder.  The disorder, aplastic anemia, is what happens when your body stops producing enough new blood cells, and can result in full body failure if it is not caught and treated early enough. 
If all of this happens, why use it?
It is relatively non-toxic to horses, taking over 90 grams to be a lethal dose for the average equine.  Consider this: Horses can weigh up to ten times as much as we do! While the toxic effects can still occur, dosages are generally going to be low enough and not frequent enough to cause damage. As with any medication, using it on an as-needed basis is unlikely to result in significant side effects.
So how does it work?
Similar to aspirin, bute is a synthetic drug that works on blocking certain enzymes that are responsible for smooth muscle cells to constrict or dilate, increase blood clotting, and increasing spinal nerve activation of pain sensation. [2,4] It is not selective to the two types of receptors, and does not bind permanently like aspirin does. [1,2] It is gone from the system typically in two to three days.
Cool, so how would I give it to my horse?
Bute comes in either oral or IV form.  While IV is certainly fastest, only someone trained in IV should give it. This is because it can cause death of the tissue if it goes outside of the blood vessel.  Oral methods are paste, tablets or powder. Find out which one your horse likes best (mine enjoys the powder on a hot mash) and be sure to follow your vet’s instructions.
 Drug Bank. (2013). Phenylbutazone. Retrieved February 10, 2015, from http://www.drugbank.ca/drugs/DB00812
 Fraser, C. M. (1986). The Merck veterinary manual (No. Edition 6). Merck & Co.
 Bishop, Y. M. (Ed.). (2005). The veterinary formulary. Pharmaceutical press.
 Small Molecule Pathway Database. (n.d.). Phenylbutazone action pathway. Retrieved February 17, 2015, from http://smpdb.ca/view/SMP00701?highlight[compounds]=DB00812&highlight[proteins]=DB00812
 Thal Equine, LLC. (2013). Bute & Banamine®: Commonly Used & Misused in Horses. Retrieved February 21, 2015, from http://thalequine.com/bute-and-banamine-what-horse-owners-should-know/