Drugs can be introduced into the bloodstream by a number of different routes. Most commonly, they are taken orally in the form of pills, capsules, or liquids. However, if oral treatment is inappropriate, drugs may be given in various other ways. For example, a suppository containing a drug can be inserted into the rectum. If an immediate effect is needed, intravenous injection or infusion ensures rapid delivery of the drug. A long-lasting effect may be achieved by administering a drug in a skin patch or as an implant inserted just under the skin.
Drugs in a nasal spray, in a sublingual tablet (placed under the tongue), or in a buccal tablet (placed inside the cheek) are absorbed through the thin mucous membrane directly into the bloodstream.
Drugs in pill, capsule, or liquid form are swallowed and pass into the digestive system. The drugs are then broken down in either the stomach or the intestines and are absorbed in the same way as food.
An injection into a blood vessel allows a drug to take effect very quickly. The drug enters the bloodstream directly and is rapidly circulated to the organ or tissues where it is needed.
Drugs are injected into a muscle in the upper arm, thigh, or buttock and then disperse into the bloodstream.
Drugs implanted or injected into fatty tissue just below the skin disperse slowly into the bloodstream.
Drugs are released continuously from an adhesive patch or a gel on the skin surface and pass through the skin into blood vessels.
Drugs inserted into the rectum in suppositories, enemas, or foam are quickly absorbed by blood vessels in the rectal wall.
Drugs that are taken orally are absorbed in the intestines and pass through the liver before entering the general circulation. Drugs that are administered by other routes enter the bloodstream before they pass through the liver. Once a drug has entered the bloodstream, it circulates to the site where its action is needed. Most drugs are metabolized (broken down or transformed) each time they pass through the liver. They are eventually excreted by the kidneys in urine and/or by the liver in bile, which is excreted in faeces.
How drugs circulate
Oral drugs pass through the liver, where some may be inactivated before reaching their site of action. Drugs given by other routes bypass the liver initially.