Background: Illegible handwriting on medical orders and drug prescriptions can have serious adverse consequences. The present study was performed to evaluate illegible writing based on an example of erroneous prescription interpretation and to discuss precautions for avoiding this situation.,Methods: A 65-year-old woman underwent an operation for mitral valve failure in 2005 and was given a prescription at the time of discharge. She obtained the prescribed anticoagulant, Coumadin (warfarin sodium), from a drugstore and complied with the medication instructions. On finishing her tablets, she visited a different drugstore to refill the prescription. However, instead of Coumadin, she was given the peptic ulcer drug Famodin (famotidine). The legibility of a copy of the initial prescription for Coumadin was evaluated at 58 pharmacies in the city center of Tokat, Turkey.,Results: The prescription was interpreted by 45 pharmacists, 52 experienced apprentices, and 16 novice apprentices as "Famoser" (famotidine) (n = 1, 0.9%), "Famodin" (n = 32, 28.3%), or "Coumadin" (n = 80, 70.8%). Among these 113 participants, 43.7% of the novice apprentices, 75% of the experienced apprentices and 75.6% of the pharmacists, supplied the correct drug, Coumadin.,Conclusion: Illegible handwriting may increase the risk for malpractice and cause harm to patients; thus, efforts should be made to ensure that handwriting on medical documents is legible.