OBJECTIVE: According to a law recently enacted in Germany “aut idem” is an imprint on the prescription obliging the pharmacist to give the patient a drug out of a group of its cheapest generic versions, unless the physician excludes this by marking “nec aut idem” on the prescription. In view of the Statuary Health Insurance spendings on pharmaceuticals the Ministry of Health thus intends to achieve savings of 250 Million Euro p. a. but doubts are manifold. To analyse the efficiency of “aut idem” a model calculation was made for epilepsy treatment. METHOD: In a decision tree model the costs of “aut idem” versus “nec aut idem” prescription of Carbamazepin were calculated. “Nec aut idem” brings no change for the patient whereas “aut idem” effects a drug switch with multiple risks for the patients. On the basis of up-to-date literature and official data sources assumptions were made on the reduction of drug efficacy due to modified bioavailability, a risk for 15% of the patients, with additional fits leading to an increase in treatment expenses of which only the costs for the patients’ social health insurance were taken into consideration. RESULTS: The What-if-analysis gives proof that in the ratio of €114,93 to €56,18 “aut idem” is more expensive than “nec aut idem” per patient and quarter year. The sensitivity analysis shows that “aut idem” will only lead to cost savings if more than 92% of the pharmacists inform the practitioners about the substitution and they consequently check the medication. CONCLUSION: With a conservative estimate of 150,000 patients in Germany suffering from epilepsy and treated with Carbamazepin “aut idem” could lead to an increase in expenses of €36 million p. a. Taking into account that there are another 16 indications such as diabetes and cardiac diseases which “aut idem” could also bring additional expenses about the cost saving effect of “aut idem” is truly to be doubted.