Q: What can happen if a horse overdoses on Previcox? My 10-year-old gelding has mild navicular disease and has been on Previcox for a few months, but he recently ran out and it took a few weeks to have more delivered. When we got the pills I restarted him with ¼ of a tablet and cut the rest of the tablets into quarters to leave at the barn. However, after nine days he was accidentally given the rest of the pills that were in the container, which was about 30 pills. The next day I hand-walked him, and he seemed perfectly fine, playful even. Are there any repercussions that he could suffer from consuming too much Previcox? —Alexa, via e-mail

A: Thank you very much for this question, which highlights several important issues in equine medicine. First, it is important to know more about Previcox (generic name firocoxib). This medication is a non-steroidal anti-inflammatory drug (NSAID) similar to phenylbutazone (Bute) and flunixin meglumine (Banamine) that’s FDA-approved for use in small animals. These drugs target enzymes called cyclooxygenases (COX-1 and COX-2) that produce hormonelike prostaglandins, which are involved in inflammation and pain as well as organ function.

Unlike phenylbutazone and flunixin, Previcox preferentially targets COX-2, which is predominantly associated with pain and inflammation, while having a lesser effect on COX-1 that produces prostaglandins involved in normal bodily functions. Thus, the reason manufacturers have developed COX-2 inhibitors such as firocoxib is to avoid side effects associated with NSAID blockage of COX-1. However, at high doses, firocoxib likely loses its ability to target only COX-2 and may have side effects as a result of inhibiting COX-1.

The principal organs that NSAIDs affect are the gut and the kidneys—so this is where you would look for problems in your horse. In the gut the most problematic complication of NSAID overdose is right dorsal colitis (damage to the large colon), which is accompanied by lower protein levels in the blood and thickening of the colon on ultrasound. Blood work and urinalysis are very helpful for detecting kidney damage; I would suggest that your veterinarian run these tests and look for a buildup of waste products in the bloodstream as well as increased protein in the urine