Enteral administration is an administration of drugs through the gastrointestinal tract. Methods of administration include oral, sublingual, and rectal. Many medications, such as tablets, capsules or drops, are usually taken orally. Oral administration is the most commonly used for routine administration due to its simplicity. The bioavailability of these drugs varies while enteral administration is effective for drugs with moderate and high oral bioavailability. In addition, oral drugs must be acid-stable. The parenteral administration is an administration that is not enteral Parenteral routes, comprise intravenous, subcutaneous, intramuscular, inhalation, and percutaneous. These routes require the use of needles or catheters. Parenteral administration is applied when drugs have low oral bioavailability. It is also applied to patients who cannot take oral drugs or rapid effect is required. Intravenous administration provides rapid achievement of effective drug concentration as there is no absorption phase. It may be used to maintain the constant level of drug in the blood. However, the plasma concentration of these drugs rapidly decreases due to biotransformation processes. Intramuscular and intravenous administration of drugs has a more continuous effect as there is an absorption phase. The concentration of the drug in the blood increases gradually. As a result, the half-life of these drugs is higher compared to intravenous drugs. Oral drugs are characterized by the lowest rate of metabolism as its absorption is relatively slow while the concentration of the drug in the blood increases gradually.