How medicines work

Medicines do not simply know where to go. Most act by interacting with a target, such as a receptor or enzyme. Others replace a missing substance or act through physical or chemical properties. The same mechanism can produce intended and unwanted effects, so dose, route and individual context matter.

What a medicine contains A medicine is used to prevent or treat illness, relieve symptoms, replace something the body lacks, or change a body process. Vaccines are medicines used to prevent particular infections. The active ingredient is the part intended to produce the biological effect. A medicine may also contain other ingredients that provide its shape, stability, flavour or delivery form. Different brands can contain the same active ingredient. Similar looking tablets can also contain entirely different active ingredients, so appearance alone cannot identify a medicine safely. Mechanism of action A mechanism of action explains how a medicine produces an effect. It links the medicine's first relevant interaction with the changes that follow in cells, tissues or microorganisms. Most medicines interact with one or more molecular targets. Common targets include receptors, enzymes, ion channels and transporters. Some mechanisms are well established, while others remain partly understood. Not every medicine works by attaching to a molecular target. Antacids neutralise stomach acid, while osmotic laxatives bring more water into the bowel. These are mainly chemical or physical actions. Receptors can be activated or blocked A receptor is a protein that responds to a signal. The body normally uses receptors to respond to substances such as hormones and chemical messengers. An agonist activates a receptor or supports its activity. An antagonist blocks a receptor or reduces the effect of the body's usual signal. For example, salbutamol activates particular receptors in airway muscle. This helps the muscle relax and can make breathing easier for someone prescribed it for an appropriate condition. Receptors are not simple switches in every situation. Medicines may activate them partly, affect several receptor types or produce different effects in different tissues. Medicines can work in several other ways Some medicines inhibit an enzyme, reducing a chemical reaction. Others replace a hormone, vitamin, enzyme or clotting factor that the body does not make in sufficient amounts. Antimicrobial medicines act on microorganisms. Some kill susceptible microorganisms, while others slow their growth or prevent an essential process. Different antimicrobials act on different organisms and targets. Other medicines change ion movement, transport substances across cell membranes, alter immune activity or affect which genes a cell uses. One medicine can have more than one mechanism of action. A biological effect is not the same as a guaranteed benefit The therapeutic effect is the intended helpful result. A medicine may clearly affect its target without producing the same clinical benefit for every person. The diagnosis and treatment aim must match the mechanism. A medicine that lowers blood pressure will not treat every cause of dizziness, and a painkiller will not remove every cause of pain. Preventive medicines may reduce future risk without producing a noticeable daily feeling. Lack of an immediate sensation does not prove that the medicine is ineffective. Dose changes the response A dose is the amount of medicine used at one time or over a stated period. The relationship between dose and effect differs between medicines. A dose may be too low to produce enough benefit. Increasing it may strengthen the effect up to a point, but further increases may add little benefit while increasing harm. More medicine is therefore not automatically better. The appropriate dose depends on the indication, formulation, age, body size, pregnancy, organ function, other medicines and individual response. Selectivity is a preference, not a guarantee Selectivity means that a medicine affects one target more strongly than others at a particular exposure. It does not mean that the medicine acts nowhere else. An unwanted effect can occur because the intended target is also present in another tissue. It can also occur because the intended effect becomes too strong or the medicine affects another target. Selectivity can change with dose. A medicine that is relatively selective at a lower exposure may affect additional targets when exposure rises. A medicine described as targeted is therefore not automatically harmless. Benefit and risk still depend on the person, dose and clinical purpose. Route changes where and how quickly a medicine acts The route is how a medicine is given, such as by mouth, inhaler, cream, injection, eye drop or patch. Route affects how quickly the active ingredient arrives and which tissues receive it. Some routes aim for a mainly local effect. A cream may act in the skin, and an inhaler can place medicine close to the airways. Some of the medicine may still enter the bloodstream. Other routes aim for a systemic effect through the bloodstream. A swallowed medicine may be absorbed from the gut, while an intravenous medicine enters the circulation directly. Not every swallowed medicine must enter the bloodstream to work. Some act within the stomach or bowel. Changing the route or formulation can alter absorption and safety. Do not crush tablets, open capsules or change how a medicine is used without appropriate advice. The body also changes the medicine How a medicine acts on the body is sometimes called pharmacodynamics. What the body does to the medicine involves absorption, distribution, metabolism and excretion. These processes determine the exposure reaching the target and how long it lasts. Liver or kidney impairment matters only when it affects the handling or safety of that particular medicine. Reduced kidney function does not mean that every medicine needs a lower dose. Depending on the medicine, the dose may stay unchanged, be reduced, be given less often or be replaced. Benefit and risk are individual Choosing a medicine means matching its mechanism to the treatment aim and considering likely benefit, possible harm and practical use. The same medicine and dose can affect different people differently. People should receive understandable information about what a medicine is for, how to use it and what to do if a dose is missed. Medicine specific instructions matter because the correct response is not the same for every product. Do not use another person's medicine or change your own dose because someone else takes a different amount. Ask the prescriber or pharmacist when instructions, effects or safety advice are unclear. When urgent help is needed Call 999 for sudden swelling of the lips, mouth, throat or tongue, severe breathing difficulty, collapse, unresponsiveness or a seizure after taking a medicine. A possible overdose or extra dose needs urgent advice even when the person initially feels well. Contact NHS 111 immediately if you are unsure what to do, and call 999 for severe symptoms. This lesson explains general pharmacology. It cannot show whether a specific medicine is suitable, effective or safe for an individual person.

A medicine's mechanism of action explains how it produces a biological effect. Clinical benefit also depends on the dose, route, exposure, treatment aim and individual person, while selectivity never means complete exclusivity.