drug interactions

Question 1

when does a drug interaction occur?

Answer 1

A drug interaction occurs when a substance alters the expected performance of a drug
- substance eg drug, food, toxin

Question 2

define a pharmacodynamic drug reaction.

Answer 2

occurs when drugs have an effect on the same target or physiological system

Question 3

define a pharmacokinetic drug reaction.

Answer 3

occurs when a drug affects the pharmacokinetics (absorption, distribution, metabolism or excretion) of another drug

Question 4

outline and explain the interactions of a pharmacodynamic drug reaction.

Answer 4

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

Question 5

list the sites in the body where drug interactions can occur.

Question 6

Explain the role of cytochrome p450 liver enzymes in drug interactions and outline the clinical consequences of enzyme induction and enzyme inhibition.

Question 7

List appropriate sources of information on drug interactions to support safe prescribing.

Question 8

Summarise how drug interactions can be predicted and avoided.

Question 9

Outline options for the management of interacting drugs.

Question 10

what are the 2 methods of drug interaction?

Answer 10

• pharmacodynamic
• pharmacokinetic

Question 11

outline the differences of synergistic and antagonistic drug interactions. when acting on receptors and on physiological system

Answer 11

• 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

Question 12

explain how pharmacokinetic drug reactions affect absorption

Answer 12

• 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

Question 13

how can drug interactions affect the drugs?

Answer 13

alters:
- absorption
- distribution
- metabolism
- excretion

Question 14

explain how pharmacokinetic drug reactions affect distribution

Answer 14

• when interactions occur and they bind, these can not pass from interstitial fluid to blood, fat or intracellular fluid

Question 15

explain how pharmacokinetic drug reactions affect metabolism

Answer 15

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

Question 16

explain how pharmacokinetic drug reactions affect excretion

Answer 16

• 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

Question 17

name an important drug-food interaction.

Answer 17

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

Question 18

as a drug prescriber how do we identify and avoid these drug interactions.

Answer 18

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

Question 19

name drug interaction resources.

Answer 19

• top 100 drugs
• BNF
• EMC - electronic medicines compendium
• electronic clinical systems - sometimes
• pharmacist
• credible meds

Question 20

how do you manage drug interactions and what will inform your approach?

Answer 20

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

Question 21

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

Answer 21

morphine

drug-drug interaction both competing for his opiod receptors

mechanism of this drug interaction:
- duplicated antagonist at opiod receptors