You probably won't be in the horse world too long before you hear about the drug phenylbutazone or 'bute' as it's often called (there are also trade names). It's use is very common in equine veterinary practice. Bute is a non-steroidal anti-inflammatory drug (NSAID). It is similar to the human drug aspirin.
Bute is a prescription veterinary drug and therefore you can only get it for your horse from your veterinarian. Vets typically prescribe Bute for orthopedic issues such as arthritis, laminitis, degenerative joint disease, lacerations, soft tissue injuries, muscle soreness, and navicular disease.
Bute works by being absorbed from the stomach (it is typically given as an oral medication) into the blood stream. There it goes to the liver and is metabolized. The metabolites then prevent the body's natural inflammatory chemicals from reaching their receptors, thereby blocking inflammation and some pain.
There are some risks to using Bute, especially long term. Bute can cause gastric ulcers and other gastrointestinal problems, blood disorders, and kidney and liver damage. Bute should not be given to horses that are very sick and/or dehydrated.
Bute can be administered as a paste that can be put directly in the back of the horse's mouth similar to paste wormer, powder that can be sprinkled on feed, pills that can be crushed, or it can be injected into a vein. The most common way horses are given bute is powder or crushed pills that can be hidden in food, and perhaps mixed with applesauce or molasses to hide the bitter after-taste.
The use of Bute in performance horses must be used with caution. Many sports such as distance riding will not allow a horse to compete if Bute has been administered. Rules vary, so it's important to check, and be aware if your horse may be subject to drug testing. Also consider the ethics of riding a horse that is on Bute.