Metformin is an insulin-sensitizing drug that is used to treat metabolic syndrome in people. It also suppresses glucose release from the liver. In the past it has received mixed reviews in horses.
A case report from February 2005 in the Journal of the American Veterinary Medical Association used metformin in combination with glyburide in an insulin-resistant and diabetic (elevated blood sugar) horse and reported that the blood sugar normalized.
A 2006 study (Vick et al, Reprod Fertil Dev) used it in obese, mildly insulin-resistant mares. They found improvement at a three grams/day dose for the first month it was used, but the effect disappeared after two months. Dosages of six or nine grams/day, instead of three, weren’t effective.
A third study was just released in the Equine Veterinary Journal. A research team from the Liphook Veterinary Hospital and The Animal Health Trust in the United Kingdom did a field study of the response to 15 mg/kg of metformin twice a day (equivalent of 7.5 grams for an 1100-lb. horse), in insulin-resistant horses and ponies with a history of laminitis. All test animals were on a forage-only diet, but actual sugar and starch levels were not tested.
In the first week of the study, there was a dramatic drop in insulin but as time went on the insulin were starting to creep back up again and four of the trial horses developed acute laminitis during the study. The only side effect seen was a transient drop in blood glucose that occurred three hours after dosing during the early stages of the trial, but this was not associated with any symptoms.
Even in humans treated with this drug, diet and exercise are the most important parts of a treatment plan for insulin resistance. However, the dramatic early response found in this study suggests this drug may eventually prove useful at least for helping to get rapid control of high insulin levels in acutely laminitic animals. The jury is still out regarding whether it will prove useful long term, but the development of four acute laminitis cases in the 18 study horses (22.2%) and the fact decreasing effects over time have now been reported in two separate studies is warning that you can’t rely on this alone.
Also note only a limited number of horses have been given metformin to date, far too few to accurately determine side-effect risks. Intestinal tract upset is the most commonly reported human side effect. Rare, but potentially fatal, lactic acidosis has also been reported. This is most likely with high blood levels caused by either overdosage or accumulation of the drug after multiple doses.
Drug accumulation occurs if a dose is given before a significant amount of the previous dose has been metabolized. There is no information available as yet on how horses metabolize this drug.
Generic metformin costs about $2 per gram from discount online pharmacies, so $30/day for an 1100-lb. horse at the dosage used in the most recent study.