This single-case study was conducted on a 34-year-old woman diagnosed with therapy-resistant depression and co-occurring atypical autism. The subject had been kept on the same medications for eight years despite her condition not improving and at the same time experiencing side effects. Previous studies and patient experiences suggest that many physicians are reluctant to end prescribed medication even if the patient is experiencing inadequate benefits and questionable effects. Co-occurring diseases often share overlapping symptoms, which can make accurate diagnosis and treatment more challenging, such as for patients with depression and autism. The problem becomes even more complicated when looking into the long-term treatment of depression occurring alongside autism spectrum disorder (ASD). The focus of conducting this case study was to determine the effect of DAT on a patient with confirmed therapy-resistant depression and ASD and if DAT would provide long-term benefit for the subject. The study’s results indicate that the patient experienced both quick improvement and long-term positive outcomes of DAT and is now in her 10th-year symptom-free.