02 How Medicines Work in the Body

How medicines act in the body

A medicine’s effect depends both on what the body does to the medicine and on what the medicine does to the body. Pharmacokinetics describes the body’s handling of a drug; pharmacodynamics describes the drug’s effects and how it produces them. Together, these processes help explain benefits, side effects, and risks.

Pharmacokinetics and ADME

ADME describes four parts of pharmacokinetics: absorption, distribution, metabolism, and excretion. Each part can affect the amount of a medicine reaching its target and how long it remains in the body.

Absorption and route

Absorption is the movement of a drug from its administration site into the bloodstream. Route affects how quickly and how much drug reaches circulation. An intravenous (IV) medicine enters circulation directly; an oral medicine passes through the digestive tract, and some may be metabolized in the gut or liver before reaching circulation. This is the first-pass effect.

Absorption can also be influenced by dosage form, food, gut movement, blood flow at the administration site, and a person’s condition. For example, delayed stomach emptying may change how quickly an oral medicine takes effect.

Distribution through tissues

Distribution is the movement of a drug through the blood into body tissues. It is affected by blood flow, tissue characteristics, and how much drug binds to blood proteins. In general, only the unbound portion is readily available to act on tissues. Some medicines cross barriers such as the blood–brain barrier more easily than others.

Metabolism and interactions

Metabolism is the chemical alteration of a drug, usually by enzymes in the liver. It may inactivate a drug, activate one that was initially inactive, or produce active or harmful metabolites.

Some medicines affect the enzymes that break down other medicines. Enzyme inhibition can raise another drug’s concentration, while enzyme induction can lower it. These interactions can change treatment effects or increase the risk of adverse reactions.

Excretion and half-life

Excretion is the removal of a drug or its metabolites from the body. The kidneys are a major route; some medicines or metabolites also leave through bile and stool or other routes. Reduced kidney function can slow removal of certain drugs, allowing them to accumulate.

A drug’s half-life is the time it takes for its amount or concentration in the body to fall by about half. Half-life helps explain how long a medicine remains in the body and how often it may be given.

Pharmacodynamics and drug effects

Drugs produce effects by interacting with targets such as receptors, enzymes, or ion channels. An agonist activates a receptor, while an antagonist blocks or reduces its activation. For example, a receptor-blocking medicine can reduce the effect of a naturally occurring signaling chemical.

As the amount of drug reaching its target changes, its effect may change too. The intended benefit is the therapeutic effect; unintended effects can include mild side effects or serious adverse reactions. Every medicine has a dose range in which expected benefits are balanced against risks. Some medicines have a narrow therapeutic range, so small changes in dose or drug concentration can matter.

Why responses differ

The same dose may not have the same effect in everyone. Response can vary with several patient and administration factors:

• Age and body composition: These can affect distribution, metabolism, and clearance. With aging, liver and kidney function may slow, increasing the chance that some medicines remain in the body longer.

• Liver or kidney function: Impairment can slow metabolism or excretion, potentially raising drug exposure.

• Genetics: Inherited differences in drug-metabolizing enzymes or drug targets can alter response.

• Other medicines, supplements, food, or alcohol: Interactions may change absorption, metabolism, or drug effects.

• Health status and prior exposure: Illness, hydration, blood flow, and tolerance can influence response.

• Administration factors: Route, dose, timing, and whether the medicine is taken as directed affect the amount and timing of drug exposure.

Safe medication care

Before administration, follow the medication order and required safety checks. Verify the patient, medicine, dose, route, and time, and review relevant allergies, vital signs, laboratory results, and precautions. Consider whether kidney or liver function, other medicines, or the patient’s current condition may affect the medicine’s action. Do not crush, split, or alter a dosage form unless the directions allow it.

After administration, assess for the expected therapeutic effect and watch for side effects or adverse reactions. For example, if a medicine is expected to lower blood pressure, assess the patient as directed and report concerning symptoms or an unexpected response. Follow monitoring and reporting protocols, and seek guidance when a dose, route, or patient-specific risk is unclear.