drug interactions Flashcards
what is a drug interaction?
An interaction is said to occur when the effects of one drug are changed by the presence of another drug, food, drink or by some other environmental chemical agent.
how does the chance of interaction increase?
Chance of an interaction increases with the number of drugs prescribed
what are the two outcomes of drug interaction?
Outcome may be harmful if the interaction causes an increase in the toxicityof a drug
•But note that a reduction in the effectof a drug may be just as harmful (e.g. warfarin
what harm can interactions do?
- prevebt correct treatment
- cause harm/ discomfort to patients
- prolong or cause hospital stay
- death
- expensive tests ti be done
are interactions always serious?
Interactions are not always clinically significant (there are relatively few pairs of drugs that should alwaysbe avoided)
•Some interactions are useful and intentional:e.g. ACE inhibitors and diuretics
what are the predisposing factors to interaction
age- older more likely to be polypharmacy
co-existing diseases
race/ genetic polymorphism
what determines the behaviour of drugs in the body can be applied to drug interactions?
The same pharmacokinetic and pharmacodynamic principles
when are interactions most likely to occur?
Interactions are most likely to occur when an interacting drug is started or stopped(but beware of drugs with long half-lives e.g. amiodarone)
how long after is a reaction seen from an interaction?
some may take a period of time to occur (e.g. enzyme inducers may take 1-3 weeks until maximal effect occurs whereas enzyme inhibitors usually have an effect within 24 hours of initiation
which interactions may require a trigger?
some may require a trigger (e.g. illness –diuretics and ACE inhibitors + diarrhoea from food poisoning)
how are interactions graded in the BNF?
by level of severity
they use consistent terms
what are the levels of severity in the bnf?
Severe—the result may be a life-threatening event or have a permanent detrimental effect
•Moderate—the result could cause considerable distress or partially incapacitate a patient; they are unlikely to be life-threatening or result in long-term effects
•Mild—the result is unlikely to cause concern or incapacitate the majority of patients
•Unknown—used for those interactions that are predicted, but there is insufficient evidence to hazard a guess at the outcome
what is level of evidence?
Most interactions have been assigned a rating to indicate the weight of evidence behind the interaction
what are the levels of evidence?
Study—for interactions where the information is based on a formal study including those for other drugs with same mechanism (e.g. known inducers, inhibitors, or substrates of cytochrome P450 isoenzymes or P-glycoprotein) •Anecdotal—interactions based on either a single case report or a limited number of case reports •Theoretical—interactions that are predicted based on sound theoretical considerations. The information may have been derived fromin vitrostudies or based on the way other members in the same class act
what will each listed interaction describe?
the effect that occurs, and the action to be taken, either based on the manufacturer’s advice from the relevant Summary of Product Characteristics or advice from a relevant authority (e.g. MHRA)
