Drug interactions; a pharmacist's perspective Flashcards
Drug interactions
“An interaction is said to occur when the effects of one drug are changed by the presence of another drug, herbal medicine, food, drink or by some environmental chemical agent.” (Stockley’s Drug Interactions, 2010)
Types of Drug Interactions
• Drug – Disease – E.g. beta-blockers and asthma • Drug – Food – E.g. grapefruit juice • Drug – Drug
Drug Interaction Mechanisms
Pharmacokinetic
- Absorption
- Distribution
- Metabolism
- Excretion
Drug Interaction Mechanisms
Pharmacodynamic
- Additive
- Synergistic
- Affected by fluid level/ balance?
Why are drug interactions important?
• They occur frequently, are preventable & are generally predictable
Sources of information:
- BNF appendix 1
- Stockley’s Drug Interactions (via Medicines Complete)
- Individual summary of product characteristics (SPC)
- Martindale: The Complete Drug Reference
- Pharmacy computer systems
- Medicines Information (hospital-based)
Management of Drug Interactions
• Identify the interaction – BNF Appendix 1 (black dot) and other printed sources – Pharmacy computer systems • Assess the evidence – Theoretical or evident in practice? – Identify the mechanism • Identify the importance – Desired or adverse effect? – Frequency / likelihood (common or rare effect?)
Clinically Significant Drug Interactions
• Potential for interactions high
– Many theoretical or clinically insignificant
• Clinically significant if combination of drugs leads to an unexpected change in a patient’s condition or a further complication of their condition
Clinically significant – what to look for
1. Vulnerable patient groups – Elderly – Multiple drug therapies (poly-pharmacy) – Renal or hepatic impairment – Chronic or serious illness 2. Particular groups of drugs: • Narrow therapeutic index • Enzyme inducers • Enzyme inhibitors
Management of Drug Interactions
Options:
• Avoid combination – Either do not take at all or adjust timings • Adjust dose • Monitor patient – Clinical monitoring – Drug levels – Markers (e.g. INR) • Continue combination – If synergistic effect is required!
– Sirolimus & grapefruit juice
Plasma concentration of sirolimus increases by grapefruit juice
– Clozapine & cytotoxics
Avoid use of cytotoxics with clozapine due to possible increased risk of ventricular arrhythmias
– Theophylline & phenobarbital
Metabolism of theophylline accelerated by phenobarbital
– Tetracyclines & zinc
Absorption of tetracyclines possibly reduced by zinc (give atleast 2-3 hours apart)
– Lithium & metronidazole
Increased risk of lithium toxicity when given with metronidazole
