Oral delivery is limited by the acidity of the stomach and digestive enzymes of the intestines. These problems have stymied researchers for at least 40 years, although a new novel approach discussed below offers some hope. The Real Thing Nasal delivery into the bath towel size area of the upper airways suffers from several problems. Poor transport across the mucous membranes of the nose required either very large doses of insulin or a chemical to enhance insulin transport. When insulin alone was used to deliver 10 units into the blood, about 100 units of insulin would be required, making costs prohibitive.
Transport enhancing chemicals create nasal irritation and rhinnorhea.
And, of course, a mild cold or stuffiness would easily alter the intended insulin dose change. Compare this to Insulin inhaled into the lungs brings insulin directly into the lungs where each breath brings insulin into contact with a surface area the size of a singles tennis court. Insulin absorption into the bloodstream occurs through the thin alveolar wall found in the lung. This appears to be the most promising approach for delivery at this time. There is some concern about the effects of chronic inhalation of a growth protein into the lungs, but the large surface area over which it is spread offers hope.