Metformin has been used for many years as oral anti-hyperglycaemic agent in the treatment of type 2 diabetes mellitus either in Poland or in the world. Metformin is the most commonly prescribed agent, but acute poisonings of this agent are rare. A review of our experience indicates, that poisoning with this agent may lead to life-threatening or fatal metformin toxicity. The authors of this publication postulate, that each patient with metformin exposure should be hospitalized. Physician must be vigilant to recognize anti-hyperglycaemic agent poisonings like biguanides when hypoglycaemia and acidosis are present in laboratory results. We present patients with metformin toxicity, some of them with fatal course.