Drug - drug interactions Flashcards
What is a drug interaction?
Modification of a drugs effect by prior or concomitant administration of another drug, herb, foodstuff or drink
(Basically a drug reacting with something else that is in the body)
Drug X has effect Y
Drug Q has effect G
Drug X & Q interact with each other
Describe the clinical effect, in terms of the letters above
The effect of X & Q will not be a direct function of their individual effects
It WILL NOT BE: ‘Effect Y + effect G’
What are the 5 types of drug interactions?
Drug - drug Herbal - drug Food - drug Drink - drug Pharmacogenetic
What is the difference between the object drug & the precipitant?
The ‘object drug’ is the one whose activity is affected by a drug interaction
The agent which causes (precipitates) such an interaction is the ‘precipitant’
What are examples of drug interactions that are clinically beneficial/useful?
Treatment of hypertension
Treatment of Parkinsonism - carbidopa & levadopa
Drug interactions can affect the protein binding and bioavailability of a drug in a number of ways
Why might this lead to pathology?
Many drugs are potent with narrow therapeutic index (range)
Small changes in blood levels can induce serious toxicity
What are the 4 pharmacokinetic interactions?
ADME
Absorption
Distribution
Metabolism
Elimination
Through what mechanisms can drug interactions affect the absorption of another drug?
Altered pH
Formation of insoluble complexes
Altered bacterial flora
Altered GIT motility
How do drug interactions effect absorption, and how can this be avoided?
Generally will affect the rate of absorption, not the amount absorbed
Most can be avoided by leaving a couple of hours between administration of the 2 drugs or whaterr
What is protein-binding displacement?
Distribution
This occurs when there is a reduction in the extent of plasma protein (albumin + a1-glycoprotein) binding to a drug caused by the presence of another drug
The more unbound drug, the higher the levels of active (un-bound) drug in the blood stream
If a drug is normally 99% bound, a 1% increase in displacement means the levels in plasma double
What 2 drugs are commonly involved in protein-binding displacement?
Indomethacin & warfarin
How do drug interactions affect drug metabolism?
When the precipitant induces or inhibits the metabolism of a drug
What is a common way in which drugs can inhibit metabolism of other drugs?
(Drugs such as clarithromycin, erythromicin, cimetidine, ketocanazole, omeprazole, CCBs etc do this)
Inhibit the cytochrome P450 system in the Liver
This is where metabolism commonly occurs
What does Cimetidine inhibit the metabolism of?
Warfarin & diazepam
What does Metronidazole inhibit metabolism of?
Warfarin & alcohol
