Drug interactions

Question 1

What is pharmacokinetics and pharmacodynamics?

Answer 1

=> Pharmacokinetics
- Movement of drug throughout the body

=> Pharmacodynamics
- The effect of the drug on the body

Question 2

What are the 2 types of drug interactions?

Answer 2

=> Pharmacokinetic interactions:
- Another drug changes the concentration of the primary drug

=> Pharmacodynamic interactions:
- Another drug changes the effect the primary drug has on the body

Question 3

What is a therapeutic index?

Answer 3

• Margin of safety between the dose that is effective and the dose that is harmful

Question 4

What drugs are known to have a narrow therapeutic index?

=> Mnemonic MIND THE GAP

Answer 4

M - Morphine (strong opioids)
I - Insulin
N - Neoplasia drugs
D - Digoxin

T - Theophylline / Aminophylline
H - Hypoglycaemic agents (oral)
E - Epilepsy drugs

G - Gentamicin (Aminoglycosides)
A - Anticoagulants
P - Psychiatry drug (lithium)

Question 5

What are the 4 phases of pharmacokinetics?

Answer 5

=> A - Absorption
- Calcium reduces the absorption of Levothyroxine

=> D - Distribution
- Verapamil displaces Digoxin from its binding sites

=> M - Metabolism

• Rifampicin increases the metabolism of Warfarin
• Cytochrome P450 inducers and inhibitors tend to affect the metabolism of drugs

=> E- Excretion
- Clarithromycin reduces the excretion of Digoxin

Question 6

What are the different Cytochrome P450 inducers and inhibitors?

Answer 6

=> Cytochrome P450 inducers:

• Phenytoin
• Carbamazepine
• Barbiturates
• Rifampicin
• St John’s worl

=> Cytochrome P450 inhibitors:

• Ritonavir
• Clarithromycin
• Fluconazole
• Tamoxifen
• Grapefruit

Question 7

What can happen if B blocker and Ca blocker are prescribed together?

Answer 7

Complete Heart Block

Question 8

What is the main risk of Trimethoprim?

Answer 8

When given alongside other drugs that can cause an increase in K levels, Trimethoprim can result in Hyperkalaemia

Question 9

Drugs that act on the same physiological process can cause pharmacodynamic drug interaction. what are some examples?

Answer 9

=> Glomerular filtration:

• NSAIDs, ACEi, Diuretics
• Can result in Nephrotoxicity

=> Cardiac conduction:

• B blockers, Ca blockers, Digoxin, Amiodarone
• Can result in Heart block

=> Haemostasis:

• Warfarin, Heparin, Aspirin, Clopidogrel
• Excessive bleeding

=> Cardiac respolarisation:

• Macrolides, Quinolones, Amiodarone, Sotalol, Methadone
• Can cause QT prolongation