Metformin is a first-line oral anti-hyperglycemic agent. It decreases insulin resistance, decreases hepatic glucose output and enhances peripheral glucose uptake. Metformin is used as a monotherapy in combination with other oral hypoglycemic agents. A major side-effect of metformin is lactic acidosis. The elimination of metformin is mainly through the kidneys, and raised plasma concentrations can cause lactic acidosis. Provided there is no overdose, metformin associated lactic acidosis rarely develops in patients without co-morbidities such as renal or hepatic insufficiency, acute infection or severe dehydration. Herein, we report a case of metformin-induced metabolic acidosis occurring in a woman who was severely dehydrated after performing Haj and treated conservatively.