Drug interactions
A drug interaction occurs when a patient’s response to a
drug is modified by food, nutritional supplements, formulation excipients,
environmental factors, other drugs or disease. Interactions between drugs
(drug–drug interactions) may be beneficial or harmful. Harmful drug–drug
interactions are important as they cause 10–20% of the adverse drug reactions
requiring hospitalisation and they can be avoided.
Types of drug–drug
interactions
Interactions between drugs may be categorised by the
underlying mechanism:
Behavioural drug–drug
interactions: It occurs when one
drug alters the patient’s behaviour to modify compliance with another drug. For
example, a depressed patient taking an antidepressant may become more compliant
with medication as symptoms improve.
Mechanism: altered compliance
Pharmaceutic drug–drug
interactions: It occurs when the
formulation of one drug is altered by another before it is administered. For
example, precipitation of sodium thiopentone and vecuronium within an
intravenous giving set.
Mechanism: outside the body
Pharmacokinetic
drug–drug interactions: It occurs when one
drug changes the systemic concentration of another drug, altering ‘how much’
and for ‘how long’ it is present at the site of action.
Mechanism: altered concentration
a)
Bioavailability: absorption or first-pass metabolism
b)
Clearance: metabolism or excretion of active drug
c)
Distribution: cell membrane transport to the site of action
Pharmacodynamic
drug–drug interactions: It occurs when
interacting drugs have either additive effects, in which case the overall
effect is increased, or opposing effects, in which case the overall effect is
decreased or even ‘cancelled out’.
Mechanism: altered effect
a)
molecular signal (e.g.
receptor)
b)
physiological effect
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