Over-dosage of phenylbutazone can lead to a variety of problems including gastrointestinal tract ulceration including oral ulceration, renal medullary crest necrosis, and the syndrome of right dorsal colitis. The maximum recommended dose of phenylbutazone is 8.8 mg/kg bodyweight, divided twice daily (ie., 4.4 mg/kg bid) for 3 days. Most cases are reported in ponies and miniature horses, not because of any innate susceptibility, but rather through over-estimation of bodyweight.
Acute signs of toxicity include anorexia and somnolence, oral and lingual ulceration, diarrhoea, and oedema due to hypoproteinaemia. Fever may also be present. Blood work confirms the hypoproteinaemia. There may be a leukopenia with a primary neutropenia with a left shift and toxicity.
The ulcerative syndrome of right dorsal colitis (RDC) is associated with non-steroidal anti-inflammatory drug use, most commonly phenylbutazone. Most cases are reported to occur after prolonged or excessive courses of the drug, but some have occurred in horses that have received appropriate doses of phenylbutazone for as little as 3-5 days. Ulcerative lesions involving the right dorsal colon have also been induced experimentally in normal horses by giving 6 grams of phenylbutazone daily for 5 days in the face of reduced access to water.