Daily Tadalafil and Gastric Emptying Time in Diabetic Gastroparesis

Inclusion Criteria: * Type 1 diabetes diagnosis * Age 18 - 65 years (inclusive) * Hemoglobin A1c ≤ 10.5% within the last 4 months * Diagnosis of gastroparesis, or symptoms consistent with gastroparesis (early satiety, chronic intermittent nausea or vomiting with food intake) * Patient has gastroparesis confirmed on screening study * A female patient is eligible to participate if she is of: Non-childbearing potential defined as pre-menopausal females with a documented tubal ligation or hysterectomy; or postmenopausal defined as 12 months of spontaneous amenorrhea, or child-bearing potential with negative serum hCG prior to each gastric emptying study. Exclusion Criteria: * Active nitrate use (e.g. Cialis, Viagra, Levitra, nitroglycerin, Isordil, Imdur, amyl nitrate/poppers) * Fasting fingerstick glucose \> 250 mg/dL * History of abdominal surgery including gastric banding procedure * Patient is on chronic parenteral feeding * Patient has a history of eating disorders (anorexia nervosa, binge eating, bulimia) * Regular opiate use * Recent (last 6 weeks) history of poor control of diabetes e.g. hypoglycemia requiring medical intervention, diabetic ketoacidosis, admission for control diabetes or complications of diabetes * Acute severe gastroenteritis * The patient has participated in another clinical trial in the last 30 days. * Use of medications potentially influencing upper gastrointestinal motility or appetite within one week of the study \[e.g., prokinetic drugs, macrolide antibiotics (erythromycin), GLP-1 mimetics/analog, amylin analog\] * History or presence of clinically significant gastro-intestinal, hepatic or renal disease or other condition that would in the opinion of the investigator make the subject unsuitable for inclusion in this clinical study. * Chronic angina or NYHA class III or IV CHF * Concurrent use of ketoconazole or itraconazole * History of severe vision loss, retinitis pigmentosa, or non-arteritic anterior ischemic optic neuropathy (NAION) * History of CVA * Pregnant females as determined by positive serum hCG test * Lactating females * Uncontrolled hypertension (SBP \> 160 or DBP \> 100) * Hypotension (SBP \< 90 or DBP \< 60) * Other major medical conditions: priapism, sickle cell anemia, multiple myeloma, leukemia, active cancer diagnosis, HIV/AIDS, alcoholism, or bleeding diathesis. * Intolerance to active or inactive ingredients of Cialis, or intolerance to other PDE-5 inhibitors.