1.2. Describe the factors that affect drug absorption, metabolism, distribution and excretion

This guide will help you answer 1.2. Describe the factors that affect drug absorption, metabolism, distribution and excretion.

Different factors influence how drugs work in the body. These factors impact how drugs are absorbed, metabolised, distributed, and excreted. Each step can affect the drug’s effectiveness and safety. Understanding these processes ensures the correct use of medicines and reduces potential harm. Below is a detailed explanation of the factors affecting these four key processes.

Drug Absorption

Drug absorption refers to how a drug enters the bloodstream after it is taken. It can be influenced by both the drug itself and the individual’s body. Absorption is important because it determines the amount of drug that reaches the blood, where it can begin to work.

Factors affecting drug absorption:

1. Route of administration
   The way a drug is given affects absorption. Drugs taken by mouth (oral route) are absorbed through the stomach or intestines. Absorption tends to be slower via this route compared to injections, where the drug enters the bloodstream directly. Other routes like skin patches, inhalers, or suppositories can also influence how well a drug is absorbed.
2. Chemical properties of the drug
   Drugs dissolve in different ways depending on their composition. Solubility refers to how well a drug dissolves in water or fats. Drugs more soluble in fats are absorbed faster because they pass easily through cell membranes. Size and molecular structure also affect how quickly a drug is absorbed.
3. Food in the stomach
   Food can slow down or stop the absorption of some drugs. For example, fatty foods may delay absorption or affect how much drug enters the bloodstream. Other drugs may require food to be absorbed properly.
4. pH of the drug and the body
   The acidity or alkalinity of the stomach or intestines can affect absorption. Some drugs dissolve better in acidic environments (the stomach), while others work in a more alkaline environment (the small intestine).
5. Blood flow to absorption sites
   Areas of the body with more blood flow absorb drugs faster. For instance, drugs absorbed in the intestines (which have a rich blood supply) work faster than drugs absorbed in fatty tissues.

Drug Metabolism

Metabolism is when the body chemically changes a drug after absorption. This process occurs mainly in the liver. Metabolism converts drugs into forms that are easier for the body to eliminate. However, each person metabolises drugs at a different rate.

Factors influencing drug metabolism:

1. Liver function
   A healthy liver is vital for metabolism. If someone has liver damage or disease, the breakdown of drugs takes longer. This can cause drugs to build up in the body, leading to harmful effects.
2. Age
   Age affects how fast the body metabolises drugs. Newborns and older adults usually metabolise drugs more slowly because their liver or enzyme activity might not work as efficiently.
3. Enzyme activity
   Enzymes in the liver chemically break down drugs. Enzyme levels vary between individuals. Some people have high enzyme levels, meaning they metabolise drugs faster, while others (like those with genetic differences) may metabolise drugs more slowly.
4. Drug interactions
   Taking multiple drugs at the same time can impact metabolism. For example, one drug may speed up or slow down the metabolism of another. This can change how effective the drug is or increase risks of toxicity.
5. Lifestyle factors
   Smoking, alcohol use, and diet can affect how drugs are metabolised. For instance, heavy drinkers may metabolise some drugs differently, as alcohol may impact liver enzymes.
6. Genetics
   Some people inherit genes that change how well their liver enzymes work. Genetic differences can either speed up or slow down metabolism. This is why some drugs may work for some people and not for others.
7. Health conditions
   Illnesses such as heart failure or kidney disease can reduce drug metabolism. These conditions slow blood flow to the liver, resulting in slower drug breakdown.

Drug Distribution

Drug distribution refers to how the drug moves from the blood to different parts of the body – such as organs, tissues, and cells. Each drug distributes differently based on its properties and the body’s condition.

Factors that influence drug distribution:

1. Blood flow to organs and tissues
   Areas with higher blood flow, such as the brain, heart, and kidneys, receive drugs faster. Poor circulation means less drug reaches the area, which may reduce its effect.
2. Protein binding
   Drugs may bind to proteins in the blood, such as albumin. If a drug binds tightly to proteins, less of it is free to move into tissues and work. For example, low albumin levels (common in liver disease) can change how drugs distribute in the body and increase their effects.
3. Fat versus water solubility
   Some drugs dissolve better in water, while others mix better with fats. Fat-soluble drugs are stored in fatty tissues and may take longer to leave the body. Water-soluble drugs distribute more quickly into blood and other fluids.
4. Barriers in the body
   Some areas, like the brain, have protective barriers that only allow certain drugs to enter. This barrier is called the blood-brain barrier. Drugs that cannot pass through this barrier won’t work in the brain, no matter how much is given.
5. Body composition
   The amount of fat, water, and muscle in a person’s body can affect drug distribution. For instance, people with higher fat content may store fat-soluble drugs for longer periods, while water-soluble drugs may distribute more quickly in leaner individuals.
6. Pregnancy
   Drug distribution changes during pregnancy. Drugs can sometimes cross the placenta and reach the baby’s bloodstream, which needs to be carefully managed during treatment.
7. Health conditions
   Diseases such as kidney or liver failure can lead to fluid retention or protein imbalance. These conditions change how drugs are distributed in the body.

Drug Excretion

Excretion refers to how the body removes drugs and their by-products. The kidneys are the main organs for excreting drugs through urine. Other methods of excretion include removal via bile, sweat, breath, or faeces.

Factors affecting drug excretion:

1. Kidney function
   Healthy kidneys excrete drugs efficiently. Kidney damage slows excretion, meaning drugs stay in the body longer and may accumulate to harmful levels.
2. Age
   Infants and older adults excrete drugs more slowly. This can lead to a build-up of drugs in the body, increasing side effects and toxicity risks.
3. pH of urine
   Some drugs are excreted faster in acidic urine, while others are removed more quickly in alkaline urine. Certain foods or medications can change urine pH, affecting excretion rates.
4. Drug properties
   Fat-soluble drugs may re-enter the liver instead of being excreted, slowing removal from the body. Water-soluble drugs are generally excreted faster.
5. Other excretory organs
   Although less common, the lungs excrete some drugs through exhalation (e.g., anaesthetic gases). Sweat and bile can also remove small amounts of drugs. If these organs aren’t working effectively, excretion may slow down.
6. Drug interactions
   Some drugs may compete with one another to be excreted in urine. This can lead to slower excretion rates for one or both drugs, increasing the risk of side effects.
7. Health conditions
   Diseases affecting the liver, kidneys, or lungs can reduce the efficiency of excretion. This delays drug elimination and may increase toxicity risks.

Summary of Factors Across All Processes

To work properly, each drug relies on the balance of absorption, metabolism, distribution, and excretion. Factors such as age, health, genetics, and the body’s conditions all play key roles. By understanding these processes, health and social care workers can support individuals’ safety and help prevent adverse effects.