Pharmacology Definitions and Terminology

Pharmacology is the study of drugs and their interactions with living organisms. It encompasses various terms and concepts related to drug action, classification, and administration. Here are some common definitions and terms used in pharmacology:


A chemical substance that interacts with the body to produce a physiological effect, either for therapeutic purposes, diagnostic purposes, or experimental research.

The World Health Organization (WHO) defines a drug as “any substance or product that is used or intended to be used to modify or explore physiological systems or pathological states for the benefit of the recipient“. This definition encompasses a wide range of substances, including prescription medications, over-the-counter drugs, traditional medicines, and other substances that have a physiological effect when ingested or otherwise introduced into the body.


The study of the biochemical and physiological effects of drugs on the body and their mechanisms of action.


The study of the absorption, distribution, metabolism, and excretion of drugs within the body.


A drug that binds to and activates a specific receptor, thereby producing a biological response similar to that of the endogenous ligand.


A drug that binds to a receptor but does not activate it, blocking the effects of agonists.


The measure of a drug’s ability to bind to a specific receptor.


The ability of a drug to produce a maximal response after binding to its receptor.


A measure of the amount of a drug required to produce a given effect; the more potent a drug, the lower the dose required to achieve the desired effect.


The time it takes for the concentration of a drug in the bloodstream to decrease by half.


The proportion of a drug that reaches the systemic circulation after administration, reflecting the rate and extent of absorption.


The process by which the body breaks down and converts drugs into more water-soluble forms for excretion.


A biologically inactive compound that is converted into an active drug within the body, typically through metabolic processes.

Therapeutic index:

The ratio of the toxic dose to the effective dose of a drug, is used to assess the safety and efficacy of a drug.

Side effect:

An unintended and often undesirable effect of a drug, which may occur in addition to the intended therapeutic effect.

Drug interaction:

A situation in which one drug affects the activity of another drug when both are administered together, potentially altering the therapeutic or adverse effects of either drug.

Drug tolerance:

A decrease in the effectiveness of a drug over time, resulting from the body’s adaptation to the presence of the drug.

Drug dependence:

A state in which an individual requires the presence of a drug to function normally, often accompanied by withdrawal symptoms upon cessation of drug use.

Route of administration:

The method by which a drug is introduced into the body, such as oral, intravenous, or topical.


The study of how an individual’s genetic makeup affects their response to drugs, with the aim of optimizing drug therapy based on a person’s unique genetic profile.


A protein on or within a cell that binds to specific molecules, such as hormones or neurotransmitters, and mediates their effects within the body.


The rate at which a drug is eliminated from the body typically expressed as the volume of blood or plasma that is cleared of the drug per unit of time.

Dose-response relationship:

The relationship between the dose of a drug administered and the magnitude of the resulting biological effect.

Therapeutic window:

The range of drug concentrations within which a drug is expected to be effective without causing unacceptable side effects.

First-pass metabolism:

The initial metabolism of a drug by the liver after oral administration, which can reduce the bioavailability of the drug.

Steady-state concentration:

The point at which the rate of drug administration equals the rate of drug elimination, resulting in a constant drug concentration in the body.

Drug resistance:

A phenomenon in which a drug becomes less effective over time, often due to changes in the target organism or the development of compensatory mechanisms.

Idiosyncratic reaction:

An unpredictable and rare adverse drug reaction occurs in a small percentage of the population and is not related to the drug’s pharmacological action.


The interaction between two or more drugs results in a greater effect than the sum of their individual effects.

Additive effect:

The combined effect of two or more drugs is equal to the sum of their individual effects.


A substance that counteracts the toxic effects of a drug or poison.


A condition or factor that makes the use of a particular drug inadvisable or potentially harmful.

Adherence (or compliance):

The extent to which a patient follows a prescribed drug regimen, including the timing, dosage, and frequency of administration.

Drug-drug interaction:

The modification of the effect of one drug by the concurrent administration of another drug.

Drug-food interaction:

The alteration of a drug’s effect by the presence of food in the gastrointestinal tract, which may enhance or inhibit the absorption or metabolism of the drug.


The use of multiple medications by a patient, which increases the potential for drug interactions and adverse effects.

Loading dose:

A higher initial dose of a drug is administered to rapidly achieve a therapeutic concentration in the body, followed by maintenance doses to maintain the desired effect.

Maintenance is dose:

A dose of a drug is administered at regular intervals to sustain the desired therapeutic effect after the initial loading dose.

Volume of distribution:

A theoretical volume that represents the extent to which a drug is distributed throughout the body, calculated as the amount of drug administered divided by its concentration in plasma.

Drug-target interaction:

The specific interaction between a drug and its molecular target, such as a receptor or enzyme, mediates the drug’s pharmacological effect.

Inverse agonist:

A drug that binds to the same receptor as an agonist but produces an effect opposite to that of the agonist, stabilizing the receptor in an inactive state.

Partial agonist:

A drug that binds to and activates a receptor but produces a submaximal response compared to a full agonist.

Allosteric modulator:

A molecule that binds to a site on a receptor or enzyme separate from the active site, thereby altering its activity or affinity for the ligand or substrate.


A rapid decrease in the response to a drug following repeated administration resulting in a diminished therapeutic effect.


The monitoring and evaluation of the safety and effectiveness of drugs after they have been approved for use, with the goal of identifying, assessing, and minimizing risks associated with their use.

Off-label use:

The use of a drug for a condition or patient population not specifically approved by regulatory agencies, often based on clinical experience or evidence from studies outside the approved indications.

Drug discovery:

The process of identifying and developing new pharmaceutical compounds with potential therapeutic applications typically involves target identification, lead optimization, and preclinical testing.

Drug development:

The process of bringing a new drug to market, including preclinical and clinical testing, regulatory approval, and post-marketing surveillance.


The study of the cost-effectiveness of drug therapies takes into account factors such as the cost of the drug, the cost of treating side effects, and the overall impact on patient quality of life.


An inactive ingredient in a pharmaceutical product that serves as a vehicle or medium for delivering the active drug, providing stability, enhancing absorption, or improving taste.

Dosage form:

The physical form in which a drug is administered, such as tablets, capsules, solutions, or suspensions.

Parenteral administration:

The delivery of a drug into the body via a route that bypasses the gastrointestinal tract, such as intravenous, intramuscular, or subcutaneous injection.

Orphan drug:

A pharmaceutical developed to treat a rare medical condition, often with limited commercial potential due to the small patient population.


The treatment of disease through the administration of drugs.

Drug solubility:

The ability of a drug to dissolve in a particular solvent, such as water or lipid, which can affect its absorption and bioavailability.

Drug formulation:

The process of combining a drug with various excipients to create a dosage form suitable for administration, taking into account factors such as stability, solubility, and bioavailability.

Drug allergy:

An immune-mediated adverse reaction to a drug may manifest as skin rashes, respiratory symptoms, or even life-threatening anaphylaxis.

Drug schedule:

A classification system is used by regulatory authorities to categorize drugs based on their potential for abuse, addiction, or medical use, with varying degrees of restriction on their availability and prescription.


The similarity in the rate and extent of absorption between two drug products with the same active ingredient is typically demonstrated by comparing their pharmacokinetic profiles.

Fixed-dose combination (FDC):

A pharmaceutical product that contains two or more active ingredients in a single dosage form, intended to simplify drug regimens and improve patient adherence.

Controlled-release formulation (CR/SR-sustained release/TR-timed release/ER-extended release):

A drug formulation designed to release the active ingredient slowly over an extended period, providing a more consistent therapeutic effect and potentially reducing the frequency of administration.

Clinical trial:

A research study is conducted in humans to evaluate the safety, efficacy, and appropriate dosage of a new drug or treatment.


An inactive substance that resembles the drug being tested is used as a control in clinical trials to account for the psychological effects of treatment and assess the true efficacy of the drug.

Drug-induced disease (DID)/Iatrogenic disease:

A condition or illness caused by the use of a drug, either as a direct result of its pharmacological action or due to an idiosyncratic or allergic reaction.

Drug monitoring:

The process of measuring drug concentrations in the body, typically in blood or plasma, to ensure that they remain within the therapeutic window and to assess compliance with prescribed regimens.

Over-the-counter (OTC) drug:

A drug that can be purchased without a prescription, usually intended for the treatment of minor ailments or for self-care.

Prescription drug:

A medication that requires a healthcare provider’s authorization for dispensation, typically due to its potential for abuse, toxicity, or the need for monitoring and supervision during use.

Drug screening:

The process of testing a large number of compounds for potential therapeutic activity, often using high-throughput techniques in the early stages of drug discovery.

Structure-activity relationship (SAR):

The study of how changes in the molecular structure of a compound can affect its pharmacological activity is often used to guide the design and optimization of new drugs.


A molecule that binds to a specific target, such as a receptor or enzyme, to produce a biological response. In pharmacology, drugs can be considered ligands.

Drug metabolism:

The biochemical modification of a drug by enzymes in the body, typically in the liver, leads to the formation of metabolites, which may be more or less active than the parent drug.

Drug delivery system:

A technology or formulation designed to enhance the targeted delivery of a drug to specific tissues or cells within the body, potentially improving efficacy and reducing systemic side effects.


The study of genetic variations that influence an individual’s response to drugs aims to optimise drug therapy based on a person’s unique genetic makeup.

Black box warning:

A warning issued by regulatory authorities, such as the FDA, to highlight serious or life-threatening risks associated with drug use is often displayed prominently on the drug’s labelling.

Drug-eluting device:

A medical device, such as a stent or implant, releases a drug to treat a specific condition or prevent complications over time.

Monoclonal antibody:

A drug derived from a single type of immune cell is designed to bind to a specific target, such as a protein or receptor, with high specificity and affinity.

Drug addiction:

A chronic, relapsing disorder characterized by compulsive drug-seeking behavior and drug use, despite harmful consequences.

Drug withdrawal:

A group of symptoms that may occur when a person suddenly stops or reduces the use of a drug on which they have become physically or psychologically dependent.

Drug desensitization:

A process by which the response of a receptor or target to a drug decreases over time, often due to repeated exposure or high concentrations of the drug.


A biologically inactive compound that is converted into an active drug in the body, usually through metabolic processes, allowing for improved drug delivery, absorption, or targeted action.

Drug tolerance:

A decrease in the effectiveness of a drug over time, requiring higher doses to achieve the same therapeutic effect, which may result from physiological adaptations or alterations in drug metabolism.

Drug potentiation:

The enhancement of the effect of one drug by another drug, often through pharmacodynamic or pharmacokinetic interactions.

Drug antagonism:

The interaction between two drugs that results in a reduction of the overall effect, which may occur when one drug inhibits the action of another drug or competes for the same receptor or target.


The fraction of an administered drug that reaches the systemic circulation and is available to exert its therapeutic effect is influenced by factors such as absorption, metabolism, and distribution.

Plasma half-life:

The time required for the concentration of a drug in the body to decrease by half, which is used to estimate the duration of action and the dosing interval.

Investigational new drug (IND):

A drug that has not yet been approved for marketing but is being studied in clinical trials to evaluate its safety and efficacy.

Prophylactic treatment:

The use of a drug to prevent the occurrence or progression of a disease or condition is often administered to individuals at high risk or prior to exposure to an infectious agent.

Empirical therapy:

The initiation of treatment based on clinical experience or general guidelines without definitive information about the cause of a patient’s condition or the specific drug that will be most effective

Adjuvant therapy:

Additional treatment is given alongside the primary therapy to enhance its effectiveness, often used in cancer treatment to reduce the risk of recurrence or progression.

These additional terms and concepts provide further insight into the diverse aspects of pharmacology, including drug development, administration, safety, and regulation. Understanding these terms can help you better comprehend the principles and practices involved in the use and development of pharmaceuticals.

Disclaimer: This article is for informational purposes only and should not be taken as medical advice. Always consult with a healthcare professional before making any decisions related to medication or treatment.

Disclaimer: This article is for informational purposes only and does not constitute medical advice. Always seek the advice of a healthcare provider with any questions regarding a medical condition.

Leave a Reply

Your email address will not be published. Required fields are marked *