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Substances can enter the body through various means. Drugs can be smoked, snorted, inhaled, injected, swallowed as pills, absorbed through mucus membranes or applied through transdermal means (applied to the skin). Some substances can be eaten or drank, such as marijuana and alcohol. Regardless of how a substance enters the body, it is carried through the bloodstream to various body tissues, which may include the brain. The method by which a drug is administered, along with other factors, determines how quickly it may also impact how the drug affects the user.

Drugs undergo four stages within the body: absorption, distribution, metabolism, and excretion. After a drug is administered, it is absorbed into the bloodstream. The circulatory system then distributes the drug throughout the body. After the drug has had its effect, then the drug is metabolized by the body. The drug is then excreted, primarily through urine or feces.

path of drugs

How a drug travels through body

  1. Absorption
  2. Distribution
  3. Metabolism
  4. Excretion

Step 1: Absorption

Absorption occurs after the substance is administered. The National Institute of Health (NIH) lists various routes of drug administration.

  • Drugs administered orally – by eating, drinking, or swallowing pills – are absorbed through the stomach and small intestine. The drug then passes through the liver before entering the bloodstream. Substances administered this way take effect more slowly than drugs taken via other routes, such as smoking or injection.
  • Injection can be one of the fastest ways for substances to reach body tissues and organs through the bloodstream. Injection into the vein – or intravenous injection – delivers the substance directly into the bloodstream. Injection into the muscle allows the substance to be absorbed into the bloodstream through the muscle tissue, and this is called an intramuscular injection. A subcutaneous injection is an injection into the fatty tissue underneath the skin, which enables the substance to be absorbed through the fatty tissue into the bloodstream. An intradermal injection is an injection into the skin tissue.
  • Transdermal administration of illicit drugs is not as common as oral administration or injection. Substances that are administered this way are applied to the skin and then absorbed into the body. Once the drug is absorbed through the skin, it enters the bloodstream to be carried throughout the body and possibly to the brain.
  • Some drugs can be inhaled as gases. Gases penetrate the lining of the lungs very quickly, which allows the drug to enter the bloodstream.
  • Snorting drugs causes them to be absorbed through the blood vessels in the nose where they enter the blood stream. This is similar to how buccal and sublingual drugs are absorbed. Buccal drugs are placed between a person’s gums and cheek, and sublingual drugs are placed under a person’s tongue. Buccal and sublingual drugs dissolve in the mouth and are absorbed through the tissue in the mouth to enter the bloodstream.
  • The Merck Manual describes other methods of drug administration.

Source: http://www.cesar.umd.edu/cesar/drugs/heroin.asp

Step 2: Distribution

Distribution occurs via the body’s circulatory system. Once a drug has entered the bloodstream, the heart pumps the blood throughout the body, carrying the substance with it. This is how drugs can reach the brain.

Before a drug can enter the central nervous system – the brain and spinal cord – it must pass through the blood-brain barrier. This is a system of tightly woven capillaries that is designed to prevent poisons and dangerous substances from reaching the brain. Drugs that are intended to act on the central nervous system are specially designed to pass through this barrier.

Once the drug has reached the brain, it can have various effects, including the rush of euphoria or the “high” that is commonly associated with illicit drug use. Drugs accomplish this by affecting the chemicals and receptors within the brain. Many illicit drugs affect dopamine levels. Dopamine is a neurotransmitter that is responsible for feelings of pleasure and reward.

According to NIH, distribution is also responsible for the many negative side effects that drugs can have on the body. Because substances are carried to the entire body through the blood, they can have many effects that were unintended or unwanted. Illicit substances can damage the heart, liver, stomach, lungs, and other internal organs. Some of the effects of illicit drug use can be long-lasting or even permanent.

Step 3: Metabolism

Once a drug has been distributed throughout the body, it is broken down or metabolized. The amount of time a drug stays in the body before being broken down varies between substances and methods of administration. All substances that enter the bloodstream, regardless of how they are administered, are eventually carried to the liver to be metabolized. Although the liver is the primary site of drug metabolism, drug metabolism may also occur in various other tissues and organs, such as the kidneys, lungs, skin, and other sites.

According to NIH, the liver metabolizes drugs through its enzymes, which break down and transform drugs into substances that can be expelled by the body. Drugs are metabolized into simpler molecules called metabolites that can be more easily excreted, typically through urine or feces. These metabolites usually have less effect on the body. Some drugs, however, produce metabolites that can cause their own effects.

Step 4: Excretion

The last phase of a drug within the body is excretion. This is the process by which drugs exit the body, primarily via urine or feces. The metabolized drug travels from the liver to the bladder and large intestine, where waste products carry what is left of the drug out of the body.

Source: https://medlineplus.gov/ency/article/003578.htm, http://www.ndcrc.org/sites/default/files/dcr.vi__0.pdf