Sunday, 6 November 2016

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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