Mechanisms of drug interactions: CYP and P-glycoprotein induction and inhibition, herbal medicines and pharmacodynamic synergy
How drugs alter each other's absorption, metabolism and excretion (CYP and P-glycoprotein inhibition and induction, chelation, tubular secretion), why time course and fraction metabolised predict severity, complementary medicine interactions, and additive pharmacodynamic harms.
Tier 1Medical sciences concept
Written with AI assistance and fact-checked against the sources below. For exam study, not clinical decisions. How notes are made
At a glance
- Pharmacokinetic interactions change how much drug reaches its site of action. Pharmacodynamic interactions change what the drug does once there.
- Inhibition is fast (days). Induction is slow (1 to 3 weeks on and off), because it needs gene transcription.
- Severity depends on how much of the victim drug's clearance runs through the blocked pathway (fraction metabolised, fm), its first-pass loss, and its therapeutic index.
Loading the rest of the note…
Finished reading?
Sources
- Australian Medicines Handbook (interactions appendix)AU/NZAustralian Medicines Handbook Pty Ltd, 2026
- Therapeutic Guidelines: Prescribing in special populationsAU/NZTherapeutic Guidelines Ltd, 2026
- For healthcare professionals: FDA's examples of drugs that interact with CYP enzymes and transporter systemsUS Food and Drug Administration, 2023
- QTdrugs listCredibleMeds (AZCERT), 2026
- Stockley's Drug InteractionsPharmaceutical Press, current online edition
- Last updated
- 14 November 2026
What changed
- 14 November 2026 Turnstile bot check on sign-in and signed-out feedback