Metformin is an oral antidiabetic drug that improves glycemic control by enhancing the sensitivity of hepatic and peripheral tissues to insulin. This study evaluated the usefulness of metformin for treatment of diabetes mellitus in cats. There were 3 phases. Phase 1 was a dose-finding study performed in healthy cats. Phase 2 was a 3-week safety study to determine if healthy cats could tolerate the daily oral dose and administration protocol identified in phase 1, and phase 3 was a clinical trial evaluating the clinical response of diabetic cats to oral metformin treatment. There is a direct correlation between metformin dose and peak plasma metformin concentration. A dose of 25 to 50 mg of metformin twice a day seemed appropriate. Intermittent lethargy, inappetence, vomiting, and weight loss were seen. Commercially available tablet strength of metformin is 500 mg, and compounding is necessary to obtain a nontoxic dose. During the phase 3 trial, 1 of 5 diabetic cats responded to metformin. Three cats did not respond, and 1 cat died unexpectedly after 11 days of therapy.

COMMENTARY: Based on this study, there appears to be no clinical advantage of using metformin as an oral alternative to glipizide for the treatment of diabetes mellitus in cats.

Evaluation of the oral antihyperglycemic drug metformin in normal and diabetic cats. Nelson R, Spann D, Elliott D. J VET INTERN MED 18:18-24, 2004.