drug interactions Flashcards
when does a drug interaction occur?
A drug interaction occurs when a substance alters the expected performance of a drug
- substance eg drug, food, toxin
define a pharmacodynamic drug reaction.
occurs when drugs have an effect on the same target or physiological system
define a pharmacokinetic drug reaction.
occurs when a drug affects the pharmacokinetics (absorption, distribution, metabolism or excretion) of another drug
outline and explain the interactions of a pharmacodynamic drug reaction.
Pharmacodynamic interactions are either synergistic (similar effects) or antagonistic (different effects)
Due to drugs acting on the same drug receptor(s) or physiological system
Generally predictable (related to pharmacology of drug)
Highly selective drugs are less likely to be problematic
list the sites in the body where drug interactions can occur.
Explain the role of cytochrome p450 liver enzymes in drug interactions and outline the clinical consequences of enzyme induction and enzyme inhibition.
List appropriate sources of information on drug interactions to support safe prescribing.
Summarise how drug interactions can be predicted and avoided.
Outline options for the management of interacting drugs.
what are the 2 methods of drug interaction?
- pharmacodynamic
- pharmacokinetic
outline the differences of synergistic and antagonistic drug interactions. when acting on receptors and on physiological system
- synergic will have an increased effect - potentially over-doing it
- antagonostic will be competing for the same thing eg receptor so incomplete therapy
- 2 different acting - cause issues
- sometimes 2 similar which act in different ways can be beneficial because they have different routes so dont interact, just amplify response
explain how pharmacokinetic drug reactions affect absorption
- the rate isn’t an issue
- the extend it because it means the drug isn’t reaching the therapeutic levels
- drugs whoch alter the PH of the GI tract to reduce the absorption of other drugs (because ionised and unionised need certain PH to be absorbed)
- drugs forming insoluble drug complexes in the GI tract - insoluble drug complexes wont be able to be absorbed into the blood through the gut wall
- drugs which alter P-glycoprotein activity to increase or decrease the absorption of other drugs - this can increase or decrease the activity of pgp which is a protein used to remove drugs from blood
how can drug interactions affect the drugs?
alters:
- absorption
- distribution
- metabolism
- excretion
explain how pharmacokinetic drug reactions affect distribution
- when interactions occur and they bind, these can not pass from interstitial fluid to blood, fat or intracellular fluid
explain how pharmacokinetic drug reactions affect metabolism
drugs can either be enzyme (CYP) inducers or inhibitors
if an enzyme inducer they will recieve reduced treatment levels - subtherapeutic treatment failure
if an enzyme inhibitor they get increased levels which can lead to ADR toxicity
explain how pharmacokinetic drug reactions affect excretion
- when 2 drugs are competing for excretion via receptors, it leads to reduced excretion
- this can lead to toxicity if theres a build up of drug
name an important drug-food interaction.
grapefruit juice is a CYP3a4 inhibitor
- milk forms insoluble complex with drugs reducing absorption
- warfarin action can be opposed to foods with high vitamin K content and cranberry juice as this is a CYP2c9 inhibitor
as a drug prescriber how do we identify and avoid these drug interactions.
look out for high risk drugs
- obtain complete drug history
- cyp450 enzyme inducers, inhibitors and substrates
- drugs with narrow therapeutic index
- high risk / critical medicines
- new drugs - little data
look out for high risk patients
- polypharmacy
- kidney or liver impairment
- extremes of age
name drug interaction resources.
- top 100 drugs
- BNF
- EMC - electronic medicines compendium
- electronic clinical systems - sometimes
- pharmacist
- credible meds
how do you manage drug interactions and what will inform your approach?
avoid combination:
- initiate an alternative drug
- temporarily suspect interacting drug
- permanently stop interacting drug
proceed with caution
- additional monitoring eg bloods
procees
- no action required
informed approach:
- patient reference
- risk to patient
- literature
- guidencce
- advice form MDT collegues
mr K is bought to a and e by his daughter after falling at home and hurting his wrist. he has recentoyl started taking codeine fr pain relief. what regular medicines wpuld interact with codeine to increase his risk of falls?
what is the mechanism of this drug interaction
morphine
drug-drug interaction both competing for his opiod receptors
mechanism of this drug interaction:
- duplicated antagonist at opiod receptors
