Drug interactions and therapeutic drug monitoring

Question 1

Medical importance of drug interactions

Answer 1

Drug interactions form approx 7% of all adverse drug reactions. About 4% of all deaths are due to ADRs and a half of which due to drug interactions. There is a distinct chance of harming patients with certain drugs.

Question 2

Factors which may predispose a patient to drug interactions

Answer 2

Elderly, young, critically ill, complicated surgical procedures, polypharmacy, food, smoking, alcohol, herbs, chronic conditions, multiple prescribing physicians, self prescribing, narrow therapeutic range, dose.

Question 3

Drug-drug interactions

Answer 3

The modification of a drugs effect by prior or concomitant administration of another drug.
OR
When the pharmacological effect of two or more drugs given together is not just a direct function of their individual needs. Object drug - the drug whose activity is affected. Precipitant - agent which precipitates interaction.

Question 4

Mechanisms of drug interactions

Answer 4

1. Pharmaceutical; change in formualtion
2. Pharmacodynamic; antagonist, additive/synergistic, drug transport, fluid/electrolyte disturbance, indirect. The action of a drug is changed due to presence of another drug acting directly on same receptor or indirectly.
3. Pharmacokinteic; As there is marked inter-individual variation it’s possible to predict potential interactions but not to predict who will have clinically significant interaction.

Question 5

Drugs can affect…

Answer 5

ADME
Absorption; form insoluble complexes, pH, flora, GI motility, rate.
Distribution; protein-protein/binding displacement.
Metabolism; induces/inhibits metabolism of another drug.
Excretion; altered GFR or tubular secretion.

Question 6

Therapeutic drug monitoring

Answer 6

All drugs are potent with a narrow therapeutic index. This means that a small change in blood levels can induce profound toxicity therefore therapeutic drug monitoring is required.