Each route has specific purposes, advantages, and disadvantages. Oral Route Oral administration is the most convenient, usually the safest, the least expensive, and therefore the most common route. However, it has limitations. Many factors, including other drugs and food, affect how drugs are absorbed after they're taken orally. Drugs administered orally are absorbed from the gastrointestinal tract.
Absorption begins in the mouth and stomach but takes place mostly in the small intestine.
To reach the general circulation, the drug must pass through first the intestinal wall and then the liver. In contrast, drugs injected intravenously reach the general circulation without passing through the intestinal wall and liver, so they have a quicker and more consistent response. Other drugs are absorbed poorly or erratically in the gastrointestinal tract or are destroyed by the acidic environment and digestive enzymes in the stomach. Despite these limitations, the oral route is used much more frequently than other routes of drug administration. Other routes are generally reserved for situations in which a person can't take anything by mouth, a drug must be administered rapidly or in a precise dose, or a drug is poorly and erratically absorbed.