A drug passes through four basic stages as it moves through the body: 

• Absorption 
• Distribution 
• Metabolism 
• Excretion

Absorption

In order for a drug to work, it must first get into the body or the tissue. Absorption is a process by which a drug passes from the site of administration into the fluids of the body that will carry it to the site(s) of action.

The route of administration significantly affects absorption. There are two primary groups of absorption routes: gastrointestinal and parenteral. Gastrointestinal routes include any means of giving medications through the digestive system. These routes can include oral, nasogastric and rectal. The oral route involves providing the drug through the mouth, where it is then swallowed into the digestive system before being used by the body. The nasogastric route is a method of introducing medication directly to the stomach or intestine through a tube. The rectal route involves introducing the medication through the rectum, where the medication is absorbed through the mucous membrane of the rectum.

Parenteral routes are a means of introducing medications to the body by any path other than gastrointestinal. These include: sublingual, buccal, topical, inhalation and injection. The sublingual route introduces the medication under the tongue and is directly absorbed through the mucous membrane. Buccal administration is placing the medication between the cheek and the gum for absorption through the mucous membrane. The topical route involves placing medication directly on the skin for absorption. This may involve a patch, cream or ointment. Most topical medications are absorbed directly into the local tissue where they performs their action. Some topical medications are absorbed through the skin into the bloodstream. Medications may also be administered into the airways through inhalation, whereby the medication is inhaled directly into the lungs and then absorbed into the bloodstream.

The injection route has multiple points of entry including intravenous, intramuscular, subcutaneous and intradermal. Each type of injection involves administering a drug into a different part of the body. Intravenous means to introduce a medication directly into the bloodstream through a vein. Intramuscular injections introduce a medication into the muscle. A subcutaneous injection involves delivering the medication into the deepest layer of the skin. Intradermal injections introduce medications just under the top layer of the skin.

The route of administration is a significant factor in the speed of the drug action. The most common route is oral. The oral route is easiest but the effects are slower. The IV route is fastest and is the most complicated. IV medications are typically administered by licensed health professionals.

Medication aides will provide significantly more medications through the oral route than any other route. The absorption of oral medications generally follows a pattern. Oral medications are first dissolved by stomach secretions. The medications are then absorbed into the bloodstream from the small intestine. The blood carries the medication to a specific tissue where the medication acts. The medication may be transported to the liver first, where it is altered into a useable form or destroyed when not needed by the body any longer. While not all medications follow this pattern, many do.

 Factors Affecting Absorption of Drugs

If the medication is a tablet or capsule, the coating on the medication may affect absorption speed. Liquid medication breaks down faster. The presence of food in the stomach can reduce stomach upset and slow absorption. The acidity of the stomach in comparison to the acidity of the medication will also impact absorption. The “pH” is an abbreviation for potential hydrogen, a scale representing the acidity or alkalinity of a solution. If the pH of a medication is more acid, it is absorbed well from the stomach. If it is more alkaline, it is absorbed well in the small intestine. The lipid (fat) solubility (ability for a given substance to dissolve) is also a factor that affects speed of absorption. For example, alcohol-based medications are absorbed well in the stomach. Motility of the stomach can significantly impact the speed of absorption. If a person has slow motility, more of the medication may be absorbed prior to being excreted through the stool. The presence of diseases such as reflux disorders, cancer, ulcers or irritable bowel disease will also affect absorption.

Distribution

Distribution is the process of transporting a drug to the site of action. Certain drugs may be more attracted to specific tissues or organs; for example, amphetamines are very attracted to cerebrospinal fluid in the brain, which accounts for the cognitive, emotional and sensory effects of the drug. Certain drugs may pass through a lipid barrier and others may not. Examples of barriers include the blood/ brain barrier and the placental barrier. These barriers are designed as a protective mechanism to prevent chemicals from harming the brain or the fetus.

Distribution most often occurs through the circulatory system in the blood, or the lymphatic fluid. The chemical is then sent to the site where action will occur. For example, a pain pill for a headache must travel from the stomach through the bloodstream to reach the nerve receptors triggering pain before relief is felt.

Metabolism

Metabolism is the process of breaking down a chemical to release energy or to use energy. Some medications are broken down further before the body can use them. Conversion of a drug into a less active and more easily excreted form makes the drug more useable for the body. Drug metabolism happens most often in the liver, with smaller amounts occurring in the kidneys, plasma and intestinal mucosa.

Excretion

When the drug has done its job, it is removed or excreted from the body, usually by the kidneys. A drug circulating in the body has an effect until it is either changed into an inactive form or is excreted. The kidneys remove the waste products of the drug from the bloodstream and excrete them into the urine. Drugs may also be excreted from the body in stool, sweat, exhaled air, saliva and breast milk. A cumulative effect can occur if a drug is not excreted and doses accumulate in the body; for example, in kidney disease. In these cases, medications can become toxic at a relatively low dose. This can be a major concern with the elderly.