Adult diabetic patients (ages 18-65) with gastric emptying scintigraphy-confirmed delayed gastric emptying will be recruited to participate in the study. Using double-blinded methodology, study participants will be randomly assigned to one of two treatment arms: promethazine 12.5 mg three times daily for 28 days or placebo three times daily for 28 days. The primary outcome will be the change in gastroparesis symptom severity, as measured by the Gastroparesis Cardinal Symptom Index (GCSI) at four weeks compared to baseline. Participants will be seen for a clinic evaluation at weeks 0, 2 and 4, during which symptom scores, adverse events and treatment compliance will be assessed. It is hypothesized promethazine treatment will be superior to placebo in improving symptoms of gastroparesis.