Drug interactions Flashcards
How to reduce drug interactions
- Avoid
- Choose non interacting alternatives
- Reduce dose
- Monitor the patient
What should tetracycline be avoided with
Why
FE2+ salts or Ca2+
Reduced the absorption of tetracycline
Why can antacids, H2 antagonists and PPIs influence absorption of some drugs
How to overcome this
Can cause rises in pH.
pH affects drug absorption. Weak bases are less absorbed in stomach. Acids are more absorbed
–Seperate both drug intakes by several hours
What drugs cause enzyme (CYP450) induction
Which one is auto inducing (induces itself so you have to increase dose over time)
Rifampicin Phenytoin Ethanol Carbamazepine St Johns Wort
How long does it take for inducing agents to take effect
How long does it take for them to ware off
May take a week or 2 for effect
Effect may persist on stopping inducer
Which drugs are CYp450 inhibitors
- erythromycin/clarithromycin
- Psoralen from grapefruit juice
What drugs is simvastatin contraindicated with and why
Macrolides - Erythromycin/ clarithromycin
Increases risk of myopathy
What drugs interact with simvastatin and why
Amlodipine, verapamil, diltiazem
-Increases risk of myopathy
What drug does methotrexate interact with in renal elimination and why
effect of this
How to counsel patients taking methotrexate
Aspirin and NSAIDs in general
May reduce renal perfusion as there is competition for weak acid or base transports.
Can cause methotrexate toxicity
Counsel patients taking methotrexate to not take OTC ibuprofen
How can diuretics interact with digoxin
They cause hypokalaemia so increase toxicity of digoxin
which drugs should K sparing diuretics be not taken with
K supplements or ACIs which increase K+
May cause hyperkalaemia
What drug are beta blockers completely contraindicated with
Why
What drug can beta blockers be used with instead
Rate limiting calcium channel blockers
Risk of asystole (potentially fatal)
Dihydropyrimidines
What drugs can be used instead of warfarin
DOACs
What should clopidogrel be avoided with
Why
Omeprazole and esomeprazole
Both biotransformed by same CYP450 so clopidogrel not converted to active metabolite and so makes it less effective
What foods should be avoided with warfarin
Cranberry juice
