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The intent of drug administration is for the active component of the drug to reach the site of action and initiate a series of events that will culminate in a drug effect. The sum of all the processes that are entailed in the movement of the drug from the site of administration to the inside of the body is considered to be absorption. For most practical purposes, administration is considered to be complete when all of the drug has entered the circulatory system and is thus being distributed to the rest of the body. Using this definition, intravenous administration does not involve absorption since the drug is placed directly into the circulatory system.

absorption rate constant (ka)

ka defines the absorption rate constant, the rate of absorption of a drug absorbed from its site of application according to first-order kinetics, for a drug administered by a route other than intravenously.


An acid is a compound that acts as a proton donor. Thus, a compound that gives up a hydrogen ion (H+) is an acid. An acidic solution is one in which the pH (see pH) is below 7.00 and thus has a relative abundance of H+ ions. A compound may be called a weak or strong acid. This indicates that the pH at which 50% of the compound is ionised when in solution is close to 7 (weak) or close to 1 (strong)

action potential

The action potential is a brief (approx 1–10 msec) change in the electrical charge across the membrane of a nerve or muscle cell. The change in membrane potential is a result of changes in the distribution of ions (particularly Na+ and K+). Prior to an action potential the charge across the membrane is held relatively constant (e.g. –70mV); this is referred to as the resting potential. In neurons, this is primarily achieved by the slow leak of K+ out of the cell, which yields a net negative charge with the inside of the cell being negative relative to the outside. This resting potential is maintained as long as the flow of ions is not disturbed. However, if the membrane is depolarised (charge moved toward 0), this will cause voltage-dependent sodium channels to open, allowing sodium to flow into the cell and causing the inside of the cell to become positively charged. Subsequently the Na+ channels close, and there is a rapid movement of K+ out of the cell as K+ channels increase their conductance. The extracellular flow of K+ results in recovery of the negative potential across the cell membrane (repolarisation). When viewed in graphical form (mVolts vs. time) the rapid change in charge appears as a spike. The whole series of events from resting potential to depolarisation to repolarisation is referred to collectively as the action potential.

active transport

Transport refers to the movement of molecules or ions across a biological membrane (e.g. into a cell). In active transport, energy is expended to transport the molecules from one side of the membrane to another. Since energy is required, active transport implies that the transport is from an area of lower concentration to an area of higher concentration (i.e. against the concentration gradient).


A maladaptive pattern of substance use leading to clinically significant impairment or distress. A dependent individual will spend a great deal of time in drug-seeking, drug-taking and recovery activities, and will persist in taking the drug despite adverse social, occupational or recreational outcomes, and in full knowledge of the psychological and physical problems resulting from their drug-taking.


The degree to which a person takes a medication as recommended by a health care provider.


The process by which a drug adheres to a surface. Drugs can adsorb to particles or proteins.

adverse drug event

An unwanted and sometimes unintended effect that occurs while a person is taking a drug.

adverse drug reaction (ADR)

The Medicines and Healthcare Regulatory Agency (MHRA) describes an adverse drug reaction (ADR) as a response to a medicinal product which is noxious and unintended. Response in this context means that a causal relationship between a medicinal product and an adverse event is at least a reasonable possibility. Adverse reactions may arise from use of the product within or outside the terms of the marketing authorisation or from occupational exposure. Conditions of use outside the marketing authorisation include off-label use, overdose, misuse, abuse and medication errors. The reaction may be a known side effect of the drug or it may be new and previously unrecognised.



Affinity is a measure of the equilibrium constant of the reversible reaction of a drug with a receptor to form a drug-receptor complex; the reciprocal of the dissociation constant of a drug-receptor complex.


A compound that binds to and activates a receptor. Can be full, partial or inverse.

  • Full agonists have high efficacy, producing a full response while occupying a relatively low proportion of receptors.
  • Partial agonists have lower efficacy than a full agonist. They produce sub-maximal activation even when occupying the total receptor population, therefore cannot produce the maximal response, irrespective of the concentration applied.
  • Inverse agonists produce an effect opposite to that of an agonist, yet binds to the same receptor binding-site as an agonist.

Recommendations regarding the use of terms and symbols used in receptor pharmacology are provided in International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification. XXXVIII. Update on terms and symbols in quantitative pharmacology.

alkylating agent

Agents that damage DNA and prevent cells from proliferating; used to treat many different cancers. classical antineoplastic alkylating agents are melphalan and chlorambucil.


A variant form of a gene.

allosteric modulator

A compound that binds to a receptor at a site distinct from the active site (agonist binding site). Induces a conformational change in the receptor, which alters the affinity of the receptor for the endogenous ligand. Positive allosteric modulators (PAMs) increase the affinity, whilst negative allosteric modulators (NAMs) decrease the affinity.

Recommendations regarding the use of terms and symbols used in receptor pharmacology are provided in International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification. XXXVIII. Update on terms and symbols in quantitative pharmacology.


A drug that dulls the sense of pain. It differs from an anesthetic agent in that it relieves pain without loss of consciousness.


A severe rapid onset inflammatory reaction that is a medical emergency and requires immediate intervention. Often caused by allergies to insect bites and stings, food allergies and allergies to medications. Epinephrine (adrenaline) is the primary treatment for anaphylaxis.


Alternative spelling, anaesthetic. A drug that causes loss of sensation. General anesthetics cause loss of sensation and loss of consciousness. Local anesthetics cause loss of sensation by blocking nerve conduction only in the particular area where they are applied.


Anorectic drugs are appetite suppressants. Also known as anorexics, anorexigenic or anorexiant agents. Examples include amphetamine (and derivatives), methylphenidate (Ritalin), and the anti-convulsants topiramate and zonisamide, which all have weight-loss as a side-effect.


A compound that attenuates the effect of an agonist. Can be competitive or non-competitive, each of which can be reversible or irreversible.

  • A competitive antagonist binds to the same site as the agonist but does not activate it, thus blocks the agonist’s action.
  • A non-competitive antagonist binds to an allosteric (non-agonist) site on the receptor to prevent activation of the receptor.
  • A reversible antagonist binds non-covalently to the receptor, therefore can be “washed out”.
  • An irreversible antagonist binds covalently to the receptor and cannot be displaced by either competing ligands or washing.

Recommendations regarding the use of terms and symbols used in receptor pharmacology are provided in International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification. XXXVIII. Update on terms and symbols in quantitative pharmacology.

area under the curve (AUC)

AUC is the area under the plot of plasma concentration of drug (not logarithm of the concentration) against time after drug administration. The AUC is of particular use in estimating bioavailability of drugs, and in estimating total clearance of drugs. The ratio of the AUC after oral administration of a drug formulation to that after the intravenous injection of the same dose to the same subject is used during drug development to assess a drug’s oral bioavailability.


Autacoids are biological factors which act like paracrine hormones: typically having a brief duration of action near the site of synthesis. Examples are histamine and serotonin.


Pertaining to the release of signalling molecules which act directly via receptors on the same cell. For example, vascular endothelial growth factor (VEGF) produced by some types of cancer cells, acts in a autocrine manner to promote their own survival and migration.

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