NSAIDs: USES AND ADVERSE EFFECTS IN THE HORSE

FLUNIXIN MEGLUMINE Flunixin meglumine (Banamine) is frequently used in horses for the treatment of colic and endotoxemia. It has not historically been used for treating lameness, due to a perceived inferiority to phenylbutazone for musculoskeletal pain. Comparative studies in horses with naturally occurring navicular disease have shown no significant differences in the analgesic effects of flunixin and phenylbutazone, however. Similarly, both flunixin and phenylbutazone have been shown to block endotoxin induced prostanoid production and inflammation. The real benefit of flunixin in treating endotoxemia or colic may therefore lie in the availability of and ease of injection of flunixin. Flunixin is absorbed well orally. Pharmacokinetic studies of powder and paste formulations show a rapid absorption with peak plasma concentrations occurring at approximately 30 minutes, and an oral bioavailability of 86%. Many people use flunixin intramuscularly, particularly owners. This practice should be discouraged, as flunixin is highly irritating and there are reports of abscessation and clostridial myositis secondary to IM administration. As an alternative to IM injections in the horse, a recent study examined the oral bioavailability of the injectable formulation. Bioavailability of this preparation was approximately 72%, with similar pharmacokinetics to the commercially available oral preparations. This may provide a safer alternative to IM injection for horse owners. There are 2 commonly used doses of flunixin. The anti-inflammatory and analgesic dose is reported to be 1.1 mg/kg PO or IV q12h. This dose has been shown to produce a maximum effect in animal models. Lower doses (0.25 mg/kg IV q8h) have been studied, however and been shown to be effective in preventing the prostanoid production associated with endotoxin challenge, with presumably fewer side effects, and without masking the signs of colic. The flunixin was given prechallenge in those instances, and therefore, the efficacy of this dose in clinic cases is often challenged. In fact, using this dose in experimental horses that are repeatedly challenged with endotoxin causes a decrease in thromboxane production, but an increase in certain other prostanoids. Caution should be used when administering flunixin to newborn foals, due to differences in pharmacokinetics, and an increased risk for gastric and duodenal ulceration. Area under the curve, clearance, and half-life of flunixin are significantly greater in newborn foals compared to adult horses. The half-life increases from 1-2 hours in adults to 6-8 hours in foals. Based on this, dosing intervals should be longer in foals to prevent toxicity of flunixin.