Drug Interaction

Question 1

How can interactions affect absorption

Answer 1

• tetracycline and Fe2+ or Ca2+(milk)
• pH
• GI motility

Question 2

Drug interaction by MDR1

Answer 2

Question 3

Drug interaction by displacement

Answer 3

• Many drugs protein bound, but only act in free form
• Drugs compete for binding sites, resulting in displacement, transient increases in toxity but then increased elimination

Question 4

Drug interaction by CYP

Answer 4

• many drugs metabolised by multiple CYPs
• Some drugs metabolised by single CYPs; most relevant for interactions
  
  • Inhibtion
  • Induction

Question 5

Drugs that can cause Cy P450 inhibtion

Answer 5

• Cimetidine
• antifungal afens
• erythromcyin
• ciclosporin
• Psolaren from grapefruit

Immediate

Question 6

Drugs that may cause cytochrome induction

Answer 6

• Barbiturates
• Rifampicin
• Phenytoin
• Ethanol
• Carbamazepine

Question 7

Omepreazole and Clopidogrel interaction

Answer 7

• Omeprazole makes clopidogrel less effective
  
  • Due to both using same CYT P450
  • Clopidogrel no longer converted to active metabolite
  • Pantoprazole does not affect C450

Question 8

Renal interactions

Answer 8

• Competition for weak acid or base transporters
• NSAIDs and methotrexate compete for elimination
• Increased pH; increased excretion of weak acids

Question 9

Fluid and electrolyte interactions

Answer 9

• Diuretics: lead to volume depletion, when adding an ACEi increased risk of hypotension
• Diuretics: Loops and thiazide cause hypokalaemia, so increase toxicity of digoxin

Question 10

Potassium sparing diuretic interactions

Answer 10

• Increase K
• May be a problem if pt takes K supplements or ACEi (also increase K)
• Risk of hypercalaemia

Question 11

Beta-blockers and rate-limiting calclium channel blocker interactions

Answer 11

• BB and verapmil; bradycardia
• BB and diltiazem

Question 12

Warfarin interactions

Answer 12

• Narrow therapeutic window
• Many interactions
  
  • Enzyme enducers lead to failure of therapy
  • Enzyme inhibitors lead to increased bleeding
• Increased bleading with aspirin

Question 13

St Johns wort interactions

Answer 13

Enzyme inducer, avoid with

• Oral contraceptives
• Antiepileptics
• HIV drugs
• Ciclosporin
• Warfarin
• Simvastatin

Question 14

Food interactions

Answer 14

Cranberry potentiates warfarin

Grapefruit interacts with simvastatin and Ca-antagonists

Question 15

Alcohol interactions

Answer 15

• CNS depressants enhanced
  
  • TCA, antihistamines, benzo
• Antibiotics
  
  • metronidazole

Question 16

Som common alarm bells

Answer 16

Question 17

Important interactions

Answer 17