Background: Methamphetamine is commonly used worldwide. It can cause psychiatric disorders as well as cardiovascular, dermatologic, and neurological symptoms. Different hypotheses, genes, and treatment focus have been developed for methamphetamine-induced psychotic disorder, implying the different etiology of methamphetamine-induced psychotic disorder and schizophrenia or schizoaffective disorder. Case Report: A 33-year-old female patient who had been diagnosed with schizoaffective disorder for 10 years. Her symptoms were fl uctuated under antipsychotics treatment. When she voluntarily came to our hospital, her dental presentations (meth mouth), reminded us her continuous use of methamphetamine. Her diagnosis was revised from schizoaffective disorder to methamphetamine- induced psychotic disorder. Conclusion: Differentiating methamphetamine- induced psychotic disorder from primary psychotic disorder in clinical practice has been a serious challenge. Meth mouth could be a clue in correctly diagnosing methamphetamine related disorder.