To understand about drug interaction in every day practice, and to refresh our

memory to most common and important interactions

Many drug interactions are harmless and many of those which are potentially harmful only occur in a small proportion of patients

Pharmacodynamic interactions Pharmacokinetic interactions

Absorption Distribution Metabolism Excretion

Resources available

BNF web bnf.org FDA web fda,gov MHRA web mhra.gov.uk EMC web emc.medicines.org.uk CKS web cks.nhs.uk

Casescenario 1

65 years old male with known stable angina and hypertension and is on aspirin, simvastatin, verapamil and GTN spray PRN( he has not used for a year)

Recently was diagnosed with glaucoma and was started on timoptol eye drop ,he presented with dizzy spells and collapsed once

βBlockers

beta-blocker and verapamil beta-blocker and amiodarone beta-blocker and digoxin beta-blocker and diltiazem Other contraindication of BB Asthma , COPD ,PVD

A year later he was admitted with fast Atrial fibrillation and was started on digoxin 125 mcg daily and was discharged. two weeks later he came to surgery complaining ofnausea, vomiting, anorexia, dizziness, fatigue, visual disturbances, abdominal pain and diarrhea.

Calcium channel blockers (CCB)

Verapamil and digoxin CCB and Grapefruit

Now he is complaining of having knee pains which are worse at the end of the day, he has tried oral codydramol tablets which did not help much. he asks for ibuprofen ?

Aspirin and Ibuprofen COX1 inhibitor

Aspirin and Methotrexate NSAIDs delay the excretion of methotrexate

Clopidogrel and PPI inhibition of the CYP2C19 isoenzyme

Case Scenario 2

55 years old male known diabetic with IHD and hypertension, on metformin ,gliclazide , Atrovastatin , aspirin , Lisinopril, bendroflumethazide and GTN spray

He is complaining of impotence that has been going on for 6 months otherwise he is well.

phosphodiesterase inhibitors and Nitrate

KCB nicorandil

Two month later he presentedwith symptoms of lower respiratory tract infection, he is allergic to penicillin , you decided that he needs antibiotics, so he wasstarted on Erythromycin , whatwould you advice him?

Statin and myopathy Increased plasma concentration of statin e.g.

erythromycin Additive effect e.gfibrate Statin and Amiodarone Increase risk of myopathy

Statin and warfarin Increase INR

Case Scenario 3

34 years old female with known depressionand has been on fluoxetine 20 mg daily.

Recently patient diagnosed with migraine and was treated with Imigran (sumatriptan ), she has also been taken St John’s wort ,

She has been having frequent episodes of headache ,now she present with restlessness, nausea ,diarrhea, hallucinations, loss of coordination

SSRI and triptans

Serotonin syndrome (SS) Neuroleptic Malignant Syndrome (NMS)-like

reactions

Take home message

Always review medication if patient presented with new sx

Prescribe new medications only when necessary

Be familiar with important drug interaction

Is the patient taken non prescribed medication

References

www.bnf.org www.fda.gov www.mhra.gov.uk www.emc.medicines.org.uk www.cks.nhs.uk www.uptodate.com Essential revision notes for MRCP 2nd

edition