The combination of R-4-trimethylammonio-3-(tetra-decylcarbamoyl)-aminobutyrate and metformin is disclosed. Said combination of anti-diabetic drugs exerts a synergic action and allows the administration of the two drugs at doses such as to avoid or reduce the occurrence of side effects. The combination is also useful for improving the therapeutic cover far from mealtimes, and in postabsorption and fasting conditions.