Routes of Administration
Every drug must be able to reach its intended site of action. A drug that improves the pumping of blood by the heart must be able to get to the heart, while a drug used to produce sleep must reach the brain. Administration is simplest for drugs that act on the skin, such as cortisone for itching. In topical administration, a cream, oil, or liquid containing the drug can be dropped or rubbed onto the skin. The drug molecules can then easily enter the cells. Topical administration is often used for the eyes or ears.
The administration process is much more complicated if a drug is intended to act on organs inside the body, such as the heart or kidneys, or to kill bacteria infecting the body. Some drugs can move easily through the skin into the blood vessels underneath, where they are distributed throughout the bodyÑa method called transdermal delivery. This method is used for the nicotine patch, as well as hormones, and some cardiac medicines. Drugs injected under the skin (subcutaneous administration) or into muscles (intramusclar injection) enter blood vessels to be carried to their intended targets.
More frequently, to reach internal organs, drugs in pill or liquid form are administered orally through the gastrointestinal (GI) tractÑthe esophagus, stomach, intestines. Once in the GI tract, the drug molecules move into the blood vessels in the walls of the stomach or small intestine, and the circulatory system transports the drug to its intended site of action, as in swallowing an aspirin for a headache. A few drugs can enter into tissues very easily because they are extremely fat-soluble and do not need to pass into the stomach or intestine to enter the bloodstream. Nitroglycerin, used to treat angina, can be placed under the tongue, where it moves through the lining of the mouth into the underlying blood vessels and is carried quickly to the heart.
When oral administration is not possible, as in an infant or a patient who is vomiting, many drugs can be given in suppository form. A suppository is inserted into the rectum; parts of the rectal wall are rich in blood vessels to carry the drug to its site of action.
Drugs can be inhaled into the respiratory system. Some, such as asthma drugs, are given via an inhaler, a device that squirts microdroplets. Some anesthetic drugs are given in gas form, mixed with the air that the patient inhales. The drugs move from the lungs to the bloodstream and are carried to the brain.
Another route of drug administration is directly into the bloodstream through a needle inserted into a vein or artery (intravenous or intraarterial administration). This method is used when a very rapid drug effect is needed, such as in a heart attack. It is also used for drugs that do not work well when administered orally. These drugs may be destroyed in the stomach, are not able to pass through the lining of the stomach or intestines to enter the blood vessels, or go directly from the intestine into blood vessels that carry them to the liver, where they are quickly destroyed before they can reach their intended target. These drugs must be administered intravenously so that they can bypass this inactivating mechanism.