Phenylbutazone is, and will continue to be, a first-line choice for anti-inflammatory therapy. It’s a potent pain reliever. However, the fact it’s so commonly used doesn’t mean it’s risk-free.
The best-described side effects of phenylbutazone in horses are intestinal tract ulceration and kidney damage. The damage to the kidneys is primarily a long-term phenomenon, but the intestinal ulceration can occur even after a single dose and is not limited to the stomach. Ulcers may be found anywhere along the gastro-intestinal tract. Even intravenous bute can cause ulcers, since this isn’t an effect of direct irritation but rather because bute blocks the formation of the normal protective mucus lining in the intestines.
Many people also are aware of the report by Dr. L. Van Hoogmoed (University of California at Davis) that bute can depress motility in the large bowel, which could lead to a tendency for impactions. But there’s more. Consider that bute:
• Can depress bone healing (Dr. C. Rhode, Ohio State, 2000).
• Administration for as little as four days will depress both total T4 and free T4 thyroid hormone levels. The effect on total T4 lasted for 10 days after stopping bute, and on free T4 it took three days to resolve (Dr. S. Ramirez, Louisiana State, 1997).
• May slow the metabolism of the antibiotic gentamicin, and gentamicin can cause deafness or kidney damage in high doses (Dr. E. Firth, New Zealand, 1996).
• Will depress proteoglycan formation by cartilage cells in culture, which can occur in live horses as well. Caution should be used with regular phenylbutazone use for the treatment of arthritis (Dr. L. A. Beluche, Ohio State, 2001).
If that’s not enough, consider also that bute’s reactions can be unintentionally intensified by:
• The presence of acute infectious/toxic/inflammatory reactions. This situation will increase the amount of free/unbound phenylbutazone in the blood, which is the active portion of the drug. This can lead to toxic reactions even if the normal dose is used (Dr. P.C. Mills, University of Queensland 1997).
• Barbiturates used to induce general anesthesia. This leads to abnormally high levels of bute in the blood for up to six hours after it is given. Basically, using bute at the same time as any drug that shares a common enzyme pathway for metabolism in the liver can lead to elevation of blood levels of either one or both (Dr. D. B. Young at Oklahoma State, 1994).
Basic pharmacological research has also shown that phenylbutazone can become pro-inflammatory and actually cause oxidative damage if it is used when there’s circulating free hemoglobin or myoglobin. It therefore should not be given to a horse that has tied up.
We don’t want you to object if your veterinarian recommends bute, but we do want to caution you that this is a prescription drug. If you have a jar of bute “on hand” in your barn, be certain you thoroughly understand its uses. Better yet, check first with your vet.