Drug Interactions. New Zealand College of Pharmacists. Purpose of Presentation. This presentation supplements the course materials. Risk Factors for Drug Interactions. High Risk Patients Elderly, young, very sick, multiple disease Multiple drug therapy Renal, liver impairment
New Zealand College of Pharmacists
Some drugs with a low therapeutic index
One drug causes a change in patient response to another drug without altering that drug’s pharmacokinetics
Interactions that occur prior to systemic administration.
Drug A increases or reduces the excretion
(usually renal) of Drug B.
Blood levels of B fall below or rise above
normal therapeutic range.
Becomes either ineffective or toxic.
Active secretion mechanisms have limited capacity.
e.g. One acid drug may saturate the acid drug
active secretion mechanism. Another acid drug
will then be secreted less efficiently
NSAIDS can decrease renal blood flow by inhibition of renal prostaglandins.
Reduced clearance of MTX and active (toxic) metabolite
Drugs A and B both bind to the same plasma protein
……….. But compensatory mechanisms maintain free drug concentration
Patient taking timolol eye drops (for more than six months).Complains of slow heart rate and dizziness shortly after taking cimetidine tablets which he had bought OTC.