Methadone maintenance therapy (MMT) is based on biology, psychology and social medicine to reduce illegal heroin use. MMT applies low addition to replace high addition; low toxicity to replace high toxicity and long duration to replace short duration. We reported two cases about MMT in our hospital. One is hepatic toxicity induced by suspected long-term MMT and another is methadone overdose caused by medication errors. In this article we also introduced treatment of methadone overdose and suggested how to prevent similar medication errors of MMT that maybe happened in the future or in other hospitals.