k_a 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.

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.

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

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.

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.

A variant form of a gene.

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

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.

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.

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.