BACKGROUND: Phelan-McDermid syndrome (PMS) is a rare genetic disorder characterized by intellectual disability, delayed language development, autism spectrum disorders, motor tone abnormalities, and a high risk of psychiatric symptoms, including bipolar disorder. CASE SUMMARY: This report presented an 18-year clinical history of a 36-year-old woman with PMS, marked by intellectual disabilities, social withdrawal, and stereotyped behaviors. Diagnosed with bipolar disorder at the age of 18 years old, she encountered significant treatment challenges, including severe adverse reactions to antipsychotic medications in 2022, which led to speech and functional regression. Through rehabilitation and comprehensive therapy, her condition gradually improved. In 2024, after further treatment, her symptoms stabilized, highlighting the complexities and successes of long-term management. CONCLUSION: Effective management of PMS requires a thorough clinical history, genetic testing, and long-term supportive care.