HD 11.05- Drug interactions

Question 1

What are the classifications of drug interactions?

Answer 1

1. Pharmaceutical
   – incompatibility of mixing two chemically incompatible substances together (milk and tetracycline = COLLATES – none into bloodstream)
2. Pharmacokinetic
3. Pharmacodynamic

Question 2

What is a pharmacokinetic interaction?

Answer 2

Modification of action of one drug (X) by drug Y because –Y alters concentration of X reaching site of action

Mediated via changes in ABSORPTION, DISTRIBUTION, METABOLISM & EXCRETION

A drug that is highly protein bounded is more likely to get an interaction.

If another highly protein bound drug comes along, competes for the protein binding. eg warfarin and aspirin.

METABOLISM-
Coadministration of an azole antifungals and warfarin
• Azole antifungals compete for cytochrome metabolism with warfarin
• Decreased clearance of warfarin (inhibit elimination)
• Elevated blood concentrations of warfarin
• Prolonged pharmacological effects and increased dose dependent toxicity

Question 3

What is a pharmacodynamic interaction?

Answer 3

Y alters X without altering its concentration.

Can be antagonistic or synergetic

Antagonistic- ß blocker with ß agonist – salbutamol + propranolol

Synergetic- CNS depressants = Alcohol plus BDZ or antipsychotic/ opiate (FENTANYL)
Sodium Valproate and BDZ
NSAIDS and Corticosteroids = risk PEPTIC ULCERATION
Aspirin and Warfarin = impaired HAEMOSTASIS – has pharmacokinetic and pharmacodynamic interaction