Substances can enter the body through various means. Drugs can be smoked, snorted, inhaled, injected, 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 to the brain through the bloodstream. The method by which a drug is administered determines how quickly it affects the body and brain.

Drugs undergo four stages within the body: absorption, distribution, metabolism, and excretion. After a drug is administered, it is absorbed into the bloodstream. The pulmonary system then distributes the drug throughout the body. After the drug has had its intended effect, it 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 or drinking – 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 is one of the fastest ways for substances to reach the brain 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.
  • 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 to the brain.
  • Some drugs can be inhaled as gases. Gases penetrate the lining of the esophagus and lungs very quickly, which allows the drug to enter the bloodstream. Drugs that are smoked are not absorbed as quickly as gases; the particles in the smoke attach to the lungs and are then absorbed. Snorting and inhaling substances carries particles of the drug into the nose and esophagus, where the particles absorb through the skin.

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 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 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 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. All substances, regardless of how they are administered, are eventually carried to the liver to be metabolized.

According to NIH, the process by which drugs are metabolized within the liver. Enzymes within the liver break down and transform drugs into substances that can be expelled by the body. Drugs are metabolized into simpler molecules that can be easily excreted in urine, called metabolites. These metabolites usually have no effect on the body. Some drugs, however, produce metabolites that can cause their own physical symptoms.

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