Pharm - Drug Interactions

Question 1

What is a drug interaction?

Answer 1

A drug interaction occurs when the action of one drug is altered by the administration of another drug

Question 2

What sort of drug interactions are there?

Answer 2

1) Physiochemical-drug interactions due to physical properties
- Chelation (sugammadex and rocuronium)
- Neutralisation (heparin and protamine)
- Precipitation (thiopentone weak acid and suxamethonium weak base)
- Adsorption (Halothane dissolves in rubber)
2) Pharmacokinetic- one drug affects how the body handles another
- Absorption (adrenaline reduces absorption of LA by vasoconstriction)
- Distribution (Aspirin [80% PB] displaces warfarin [97% PB], so increases bioavailable warfarin and increases bleeding risk)
- Metabolism (cytochrome P450 system - PC BRAS and VIDEOCASES)
- Excretion (alkalinisation of urine increases excretion of salicylates)
3) Pharmacodynamic - action of one drug is altered by administration of another
- Summation: action of two or more drugs is additive e.g. co-administered volatile agents
- Synergism: combined action is greater than the sum of effects e.g. propofol and remifentanil
- Potentiation: action of one drug is increased by administration of another e.g. probenecid reduces renal excretion of penicillin
- Antagonism: action of one drug is blocked by another e.g. morphine and naloxone

Mechanisms of drug interactions can be direct e.g. benzos and flumazenil, or indirect e.g magnesium and non-depolarising neuromuscular blockers

Question 3

Cytochrome P450 inducers and inhibitors

Answer 3

PC BRAS - inducers
Phenytoin
Carbamazepine
Barbituates
Rifamicin
Alcohol - long term
Smoking

VIDEOCASES - inhibitors
Valproate
Isoniazid
Disulfiram
Erythromycin, clarithromycin (NOT azithromycin)
Omeprazole
Cimetidine
Allopurinol
Sulphonamides
Ethanol - acute
Steroids