Drug Interactions

Drug Interactions
Cytochrome P450 inducers
Reduce the concentration of drugs metabolised by the cytocrome P450 system.
Mnemonic: CRAPS out drugs
Carbamazepine
Rifampicin
bArbituates
Phenytoin
St Johns wort

Cytochrome P450 inhibitors
Increase the concentration of drugs metabolised by the cytocrome P450 system.
Mnemonic: Some Certain Silly Compounds Annoyingly Inhibit Enzymes, Grrrrrrr
Sodium valporate
Ciprofloxacin
Sulphonamide
Cimetidine/omeprazole
Antifungals, amiodarone
Isoniazid
Erythromycin/clarithromycin
Grapefruit juice

Examples of drugs which interact with enzyme inhibitors/inducers
Warfarin
COCP
Theophylline
Corticosteroids
Tricyclics
Pethidine
Statins

Some other common interactions to know
Metformin & cimetidine (inhibits renal elimination)
Gentamycin & loop diuretics (renal failure risk)
ACE inhibitors & potassium-sparring diuretics (risk of hyperkalaemia)
ACE inhibitors & metformin (enhance hypoglycaemic effect)
Statin & macrolides
Statin & amiodarone (increased statin concentration and therefore risk of rhabdomyolysis)
Thiazide & PPI (hyponatraemia)
Thiazide & lithium (increase toxicity)

Narrow therapeutic range drugs have many interactions (always look them up in the BNF)
Mnemonic: Guys With Large Dongles Totally Make Perfect Internet Connections
Gentamycin
Warfarin
Lithium
Digoxin
Theophylline
Methotrexate
Phenytoin
Insulin
Ciclosporin

© 2013 Dr Christopher Mansbridge at www.OSCEstop.com, a source of free OSCE exam notes for medical students’ finals OSCE revision