How Do Medicines Work in the Body?
How Do Medicines Work in the Body?
Medicines work by changing a biological process in the body or by acting on microbes or abnormal cells. To produce an effect, a medicine usually must enter the body, reach the right place and interact with a suitable target. Its effects depend on how much reaches that target, how long it remains there and how the body responds.
- What Is a Medicine and What Does It Mean for a Medicine to Work?
- What Happens to a Medicine After It Enters the Body?
- How Does a Medicine Reach the Part of the Body Where It Is Needed?
- What Are Drug Targets and How Do Medicines Act on Them?
- How Can Medicines Change the Way Cells and Organs Work?
- Why Does the Dose of a Medicine Matter?
- How Quickly Do Medicines Start Working and How Long Do Their Effects Last?
- Why Can a Medicine Produce Both Intended Effects and Side Effects?
What Is a Medicine and What Does It Mean for a Medicine to Work?
A medicine is a substance used to prevent disease, control a biological process, relieve symptoms, replace something the body lacks or treat an illness. Medicines include small chemical compounds, hormones, proteins, antibodies, vaccines and other biological products.
The part of a medicine that produces its biological effect is called the active ingredient. Tablets, capsules and liquid medicines also contain inactive ingredients that help form, protect, preserve or deliver the active ingredient.
A medicine is said to work when it produces its intended biological or clinical effect. This might mean lowering blood pressure, reducing pain, replacing a missing hormone, controlling an infection or slowing the growth of abnormal cells.
A medicine does not always remove the cause of a problem. Some medicines cure an infection, while others control a body process or reduce symptoms without eliminating the underlying cause. The meaning of “working” therefore depends on what the medicine was designed to do.
What Happens to a Medicine After It Enters the Body?
The body begins handling a medicine as soon as it is taken. Four main processes determine what happens next:
- Absorption: how the medicine enters the blood or surrounding tissue.
- Distribution: how it travels to different parts of the body.
- Metabolism: how the medicine is chemically changed.
- Excretion: how the medicine and its breakdown products leave the body.
Together, these processes are often shortened to ADME. Their study is called pharmacokinetics, which means what the body does to a medicine.
How Is a Medicine Absorbed and Distributed?
Absorption depends on how a medicine is given. A swallowed tablet must dissolve and release its active ingredient before the drug can cross the lining of the digestive tract. Most absorption from oral medicines occurs in the small intestine because it has a large surface area and a rich blood supply.
Food, stomach acidity, intestinal movement and the form of the medicine can affect how quickly and completely it is absorbed. Some medicines are poorly absorbed when swallowed, so they may be given by injection, inhalation, a skin patch or another route.
A medicine injected directly into a vein enters the bloodstream without first crossing the digestive tract. A medicine applied to the skin may remain mainly in the local tissue or gradually enter the blood, depending on how it is made.
After entering the circulation, the medicine is distributed through the blood. Some drug molecules travel freely, while others attach temporarily to proteins in the blood. Usually, only the unbound portion can easily leave the bloodstream and interact with its targets.
Distribution is not equal throughout the body. Blood flow, tissue structure, fat content and protective barriers all influence where a medicine can go. Small organs with a strong blood supply may receive a drug sooner than tissues with less blood flow.
How Is a Medicine Broken Down and Removed?
Many medicines are chemically changed by enzymes, mainly in the liver. This process is called drug metabolism. Metabolism often makes a fat-soluble drug more water-soluble so that the kidneys can remove it more easily.
A breakdown product is called a metabolite. Some metabolites are inactive, while others continue to produce an effect. A few medicines are given in an inactive or weakly active form and must be changed by the body into their active form. Such a medicine is called a prodrug.
The kidneys remove many medicines and metabolites from the blood and release them into urine. Other substances leave through bile and feces. Smaller amounts may leave through the lungs, sweat, saliva or breast milk.
The speed of removal is called clearance. Liver function, kidney function, age, genetics and interactions with other substances can affect how quickly a medicine is cleared. If removal is slow, repeated doses may cause the drug to build up in the body. (Pharmacokinetics)
How Does a Medicine Reach the Part of the Body Where It Is Needed?
Most medicines carried in the bloodstream travel to many tissues rather than directly to one organ. They produce their strongest effects where suitable targets are present and where enough of the medicine can reach those targets.
The drug’s chemical properties help determine which barriers it can cross. Small, fat-soluble molecules often pass through cell membranes more easily than large or strongly water-soluble molecules. Some drugs use transporter proteins that carry them across membranes.
Certain areas are harder to reach. The blood-brain barrier tightly controls the movement of substances from the blood into brain tissue. A medicine intended to act in the brain must either cross this barrier or use a route that avoids part of it.
Delivery can also be made more local. An inhaled medicine can place a high concentration in the airways. An eye drop acts mainly at the surface of the eye, while a skin cream can act mainly within the skin.
Local delivery does not always keep a medicine completely out of the bloodstream. Some of the dose may still be absorbed and produce effects elsewhere in the body.
Reaching a tissue is only one part of the process. The medicine must also remain chemically active, be present at a high enough concentration and come into contact with a target that can produce the intended effect.
What Are Drug Targets and How Do Medicines Act on Them?
A drug target is a molecule or structure that a medicine interacts with to produce an effect. Most targets are proteins, although medicines can also act on DNA, RNA, cell membranes or other biological materials.
Four common groups of protein targets are receptors, enzymes, ion channels and transporters.
| Drug target | Normal function | How a medicine may act |
|---|---|---|
| Receptor | Receives a chemical signal | Activates or blocks the signal |
| Enzyme | Speeds up a chemical reaction | Slows or stops the reaction |
| Ion channel | Controls the movement of charged particles | Opens, closes or blocks the channel |
| Transporter | Moves substances across a membrane | Blocks or changes transport |
A medicine works because its size, shape, charge and chemical properties allow it to interact with a target. The fit does not have to be permanent. Many drugs attach for a short time and then separate from the target.
How Do Medicines Act on Receptors?
A receptor is a protein that detects a particular chemical signal. Hormones, neurotransmitters and other signaling molecules bind to receptors and cause the cell to respond.
A medicine that activates a receptor is called an agonist. It may copy or strengthen the action of a natural signaling molecule. For example, some inhaled medicines activate receptors that relax the muscles surrounding the airways.
A medicine that prevents a receptor from being activated is called an antagonist. It may occupy the receptor without switching it on, blocking the natural signal or another drug from binding.
Some drugs only partly activate a receptor. Others change how strongly the receptor responds without occupying the usual binding site.
Receptors can be located on the cell membrane or inside the cell. Membrane receptors often produce rapid changes through internal signaling pathways. Receptors for steroid and thyroid hormones are found inside cells and can change which genes are active.
How Do Medicines Affect Enzymes?
Enzymes speed up chemical reactions in the body. They help make or break down hormones, signaling molecules, nutrients and many other substances.
A medicine can reduce an enzyme’s activity by attaching to it and blocking the area where the normal substance would bind. Other drugs attach elsewhere and change the enzyme’s shape or activity.
Blocking an enzyme can reduce the production of a harmful or unwanted substance. It can also prevent the breakdown of a useful substance, allowing that substance to remain active for longer.
Some enzyme inhibitors bind temporarily, while others form a long-lasting connection. The effect may end when the drug separates, is cleared or the cell makes new enzyme molecules.
How Do Medicines Affect Ion Channels and Transporters?
Ion channels are proteins that allow charged particles such as sodium, potassium, calcium or chloride to cross cell membranes. These movements help control nerve signals, muscle contraction, heart rhythm and the release of hormones.
Medicines can block an ion channel, keep it open for longer or change the conditions under which it opens. Local anesthetics, for example, block certain sodium channels in nerves and prevent pain signals from traveling.
Transporters bind to substances and carry them across cell membranes. They help move nutrients, salts and signaling molecules into or out of cells.
A medicine may block a transporter and allow its usual cargo to build up. Other medicines use transporters to enter cells or move through the kidneys and digestive tract. (Receptors, IUPHAR Pharmacology Education Project)
How Can Medicines Change the Way Cells and Organs Work?
The immediate action of a drug may occur at one molecule, but its final effect can involve an entire cell, tissue or organ. Changing one receptor, enzyme or channel can alter a chain of biological events.
Some changes occur within seconds. Others require hours or days because they depend on altered gene activity, the production of new proteins or gradual changes in cell numbers.
How Can Medicines Activate or Block Body Signals?
Cells communicate through chemical signals. A signaling molecule may bind to a receptor, activate proteins inside the cell and produce a response.
Medicines can enter these pathways at different points. An agonist can activate a receptor, while an antagonist can block it. An enzyme inhibitor may prevent a signal from being made or broken down. A transporter inhibitor may change how much of a signaling molecule remains outside a cell.
A medicine does not always switch a process completely on or off. It may increase or decrease the strength of a signal. The final response depends on the dose, the number of targets present and the condition of the tissue.
The same signal may have different effects in different organs. A receptor can be connected to one response in heart tissue and another response in the airways. This helps explain why a medicine acting on one receptor type can affect several parts of the body.
How Can Medicines Replace Substances the Body Is Missing?
Some medicines replace a substance that the body cannot make in sufficient amounts. The replacement may restore a normal biological signal or supply a material needed for cell function.
Insulin can replace the hormone needed to move glucose from the blood into many cells and regulate glucose production by the liver. Thyroid hormone can replace hormone that an underactive thyroid gland does not produce in sufficient amounts.
Replacement medicines can also include digestive enzymes, blood-clotting proteins, vitamins and minerals. The replacement must reach a suitable concentration, but giving more than the body needs may disturb normal regulation.
These medicines may replace the missing substance without repairing the cells or organ that normally makes it. Their effects may therefore last only while suitable amounts remain in the body.
How Can Medicines Kill or Control Microbes and Abnormal Cells?
Antimicrobial medicines act on structures or processes needed by microbes. Antibiotics may damage bacterial cell walls, block bacterial protein production or prevent bacteria from copying their genetic material. Some kill susceptible bacteria, while others stop them from multiplying and allow immune defenses to clear the infection.
Antiviral medicines can block the entry of a virus into cells, the copying of viral genetic material or the assembly and release of new virus particles. Antifungal and antiparasitic medicines target features of fungi or parasites.
Cancer medicines may damage DNA, interfere with cell division, block growth signals or help the immune system recognize cancer cells. Some traditional chemotherapy medicines affect rapidly dividing cells, while targeted medicines act on particular molecules used by certain cancers.
Perfect selectivity is difficult because microbes and abnormal cells share some biological processes with healthy human cells. The most useful targets are those that are absent from normal cells, sufficiently different or especially important to the unwanted cells. (Antimicrobials; Chemotherapy to Treat Cancer)
Why Does the Dose of a Medicine Matter?
The dose determines how much medicine enters the body. However, the amount taken is not always the same as the amount that reaches the drug target.
Absorption, distribution and removal affect the concentration at the site of action. The route of administration and the time between doses also matter.
What Is the Relationship Between Dose and Effect?
A very low dose may produce little or no measurable effect because too few target molecules are affected. As the dose and target concentration rise, the effect usually increases.
This relationship is called the dose-response relationship. It is often shown as a curve. The effect may rise steeply over one part of the curve and then level off as it approaches a maximum.
Once the maximum useful effect has been reached, a higher dose may produce little extra benefit. It may still increase unwanted effects because the drug reaches additional tissues or interacts with other targets.
The relationship is not identical for every person. Age, body composition, genetics, organ function, other medicines and the condition being treated can change the amount needed to produce an effect.
Repeated exposure may also change the response. In some cases, the same dose becomes less effective as cells or body systems adjust. This reduced response is called tolerance.
What Is the Therapeutic Range?
The therapeutic range, also called the therapeutic window, is the range of drug concentrations or doses expected to produce benefit without causing unacceptable harm.
A medicine with a wide therapeutic range has a fairly large separation between effective and harmful amounts. A medicine with a narrow therapeutic range has a much smaller separation. Small changes in dose, absorption or removal may then produce a large change in effect.
The therapeutic range is not one exact boundary that applies equally to everyone. The useful concentration may differ with the intended effect and individual biological factors.
For some medicines with a narrow range, the amount in the blood may be measured. This is called therapeutic drug monitoring. It helps determine whether the concentration is likely to be too low, suitable or potentially harmful. (FDA: E4 Dose-Response Information to Support Drug Registration; FDA: Narrow Therapeutic Index Drugs)
How Quickly Do Medicines Start Working and How Long Do Their Effects Last?
A medicine’s effects begin when enough active drug reaches its target and changes the relevant biological process. Its effects decline when the concentration falls or when the target and body system return toward their earlier state.
Some medicines act within seconds or minutes. Others take days or weeks to produce their full effect.
What Determines the Onset and Duration of a Medicine’s Effect?
The onset of action is the time between administration and the beginning of a noticeable or measurable effect. It depends partly on how quickly the drug is absorbed and distributed.
A medicine injected into a vein can reach the circulation immediately. A swallowed medicine must dissolve and be absorbed, while a specially coated or extended-release tablet may be designed to release its active ingredient slowly.
The location and mechanism of the target also affect onset. Blocking an ion channel may produce a rapid change. Altering gene activity or replacing cells and proteins may take much longer.
The duration of action is the length of time the medicine continues to produce an effect. It is influenced by how quickly the drug leaves its target, is redistributed, is metabolized and is excreted.
A drug’s half-life is the time required for its concentration in the body to fall by half. A long half-life often supports a longer dosing interval, but half-life and duration of action are not always the same.
Some medicines leave the blood quickly but produce a longer effect because they bind tightly to their targets or cause changes that take time to reverse. Others remain in the blood after their noticeable effect has declined.
With repeated doses, a medicine may gradually build up until the amount entering the body is balanced by the amount being removed. This is called a steady state.
Why Can a Medicine Produce Both Intended Effects and Side Effects?
A medicine can produce an intended effect and side effects because its target may be present in several tissues, or because the medicine interacts with more than one target.
A biological pathway may also serve several purposes. Changing it to produce one useful result may disturb another normal function.
A side effect is an effect other than the main intended one. Some side effects are mild, while others are serious. A side effect is not always caused by the drug attaching to the wrong target. It may result from a strong action on the intended target in another organ.
The amount of medicine matters. At a low concentration, a drug may act mainly on the target to which it binds most strongly. At a higher concentration, it may begin to affect additional targets.
Why Are Some Medicines More Selective Than Others?
Selectivity means that a medicine acts more strongly on one target, target subtype, tissue or organism than on others. Selectivity is relative rather than absolute.
A drug may be selective because its shape fits one receptor better than related receptors. It may enter one tissue more easily, be activated mainly within a certain organ or bind to a target found mostly in abnormal cells.
The number and distribution of targets also matter. A drug may be highly selective for one receptor, but that receptor may be present in several organs. The medicine can then produce different effects in each location.
Selectivity can change with dose. As the concentration rises, the drug may begin binding to targets for which it has a weaker attraction. This can increase side effects without adding much benefit.
Greater selectivity can reduce some unwanted effects, but it does not guarantee that a medicine will have none. Many normal systems are connected, so changing one carefully chosen target can still influence several body functions. (Pharmacodynamics, IUPHAR Pharmacology Education Project)
Written by Chris Morais, MSc, MPhil, PhD — Making complex biology simple
Disclaimer: This article is for general educational purposes only and does not constitute medical advice, diagnosis, or treatment. Consult a qualified healthcare professional about personal health concerns.
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