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Flashcards in Drug interactions; a pharmacist's perspective Deck (19)
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1
Q

Drug interactions

A
“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)
2
Q

Types of Drug Interactions

A
• Drug – Disease
– E.g. beta-blockers and asthma
• Drug – Food
– E.g. grapefruit juice
• Drug – Drug
3
Q

Drug Interaction Mechanisms

Pharmacokinetic

A
  • Absorption
  • Distribution
  • Metabolism
  • Excretion
4
Q

Drug Interaction Mechanisms

Pharmacodynamic

A
  • Additive
  • Synergistic
  • Affected by fluid level/ balance?
5
Q

Why are drug interactions important?

A

• They occur frequently, are preventable & are generally predictable

6
Q

Sources of information:

A
  • 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)
7
Q

Management of Drug Interactions

A
• 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?)
8
Q

Clinically Significant Drug Interactions

A

• 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

9
Q

Clinically significant – what to look for

A
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
10
Q

Management of Drug Interactions

Options:

A
• 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!
11
Q

– Sirolimus & grapefruit juice

A

Plasma concentration of sirolimus increases by grapefruit juice

12
Q

– Clozapine & cytotoxics

A

Avoid use of cytotoxics with clozapine due to possible increased risk of ventricular arrhythmias

13
Q

– Theophylline & phenobarbital

A

Metabolism of theophylline accelerated by phenobarbital

14
Q

– Tetracyclines & zinc

A

Absorption of tetracyclines possibly reduced by zinc (give atleast 2-3 hours apart)

15
Q

– Lithium & metronidazole

A

Increased risk of lithium toxicity when given with metronidazole

16
Q

Why are drug interactions important?

- Potential for harm/ benefit

A

•Potential for harm

  • Increasing toxicity of drugs
  • Reducing efficacy of drugs

•Potential for benefit
-Additive effects in certain
medical conditions

17
Q

• Narrow therapeutic index

A

– E.g. digoxin, theophylline, warfarin, ciclosporin, phenytoin, carbamazepine

18
Q

• Enzyme inducers

A

– E.g. carbamazepine, phenytoin, rifampicin

19
Q

• Enzyme inhibitors

A

– E.g. amiodarone, ciprofloxacin, diltiazem, fluoxetine, verapamil, ketoconazole