drug interactions Flashcards
what is a drug interaction defined as?
the modification of a drugs effect by prior or concomitant administration of another drug
when has a drug interaction occurred?
when the pharmacological effect of two or more drugs given together is not just a direct function of their individual effects
what are the drugs that are most commonly involved in drug interactions?
lithium digoxin warfarin theophylline cyclosporin phenytoin gentamicin
what increases the chance of a drug-drug interaction occurring?
higher number of medications being very old or young critically ill patients patients undergoing complicated surgical procedures patients with chronic conditions
what are the 3 mechanisms by which drug-drug interactions may occur?
pharmaceutical
pharmacokinetic
pharmacodynamic
what are some examples of pharmacodynamic interactions?
antagonistic interactions
additive or synergistic interactions
interactions due to changes in drug transport
interactions due to fluid and electrolyte disturbances
indirect pharmacodynamic interactions
what can happen in the GI tract with regards to absorption of drugs?
interactions can occur that change the rate of absorption rather than the extent ( mostly) clinically significant when drugs have short half lives or high plasma levels are required rapidly
what happens in the GI tract with regards to drug binding?
drugs can bind to each other (such as tetracyline and erythromycin complex with iron, calcium and magnesium)
why is pH significant in determining the absorption rate of a drug?
absorption is affected by the degree of ionisation which is dependent on pH
what can reduce pH?
H2 agonists, proton pump blockers and antacids
what is the effect of broad spectrum antibiotics on normal gut flora?
destroys normal gut flora - can lead to OCP failure or digoxin toxicity
where are most oral medicines absorbed?
small intestine
what is the rate limiting step of absorption of oral medicines?
gastric emptying
when does protein binding displacement occur?
when there is a reduction in the extent of plasma protein binding of a drug caused by the presence of another drug
what does displacement of a drug from plasma proteins result in/
increased bioavailability of the displaced drug
what are the two most important plasma proteins with regard to drug binding?
albumin
alpha 1 glycoprotein
when do drug interactions involving metabolism occur?
when one drug induces or inhibits the metabolism of another
where does metabolism occur?
commonly in the liver via the cytochrome p450 system
what drugs can inhibit the cytochrome system?
clarithromycin erythromycin omeprazole CCBs these drugs inhibit the metabolism of a small group of drugs metabolised by the cytochrome P450 system
what drugs can induce cytochrome P450?
barbiturates
phenytoin - warfarin, steroids
rifampicin - warfarin
tobacco
where does drug elimination occur?
the kidney
what can inhibit drug excretion?
vrapamil/diltiazem
what are the effects of loop diuretics?
increase tubular reabsorption
what can beta blockers interact with?
agonists such as salbutamol
what are synergistic interactions?
when two drugs with the same pharmacological effect are given concurrently
what do NSAIDs interact with?
antihypertensive medication and heart failure treatment
