ABSTRACT
Administration via the oral route has been, and still is, the most popular and convenient route for patient therapeutics. However, even though it is the most convenient route, it is not the simplest route, as the barriers of the gastrointestinal (GI) tract are in many cases difficult to circumvent. The main barriers of the GI tract to systemic delivery are the environment in the stomach and intestinal lumen, the presence of different enzymes, the physical barrier of the epithelium, and the liver extraction. These barriers are of functional importance for the organism in controlling intake of water, electrolytes, and food constituents, and still remain a complete barrier to harmful organisms such as bacteria, viruses, and toxic compounds.
