drug interactions

Question 1

what is a drug interaction defined as?

Answer 1

the modification of a drugs effect by prior or concomitant administration of another drug

Question 2

when has a drug interaction occurred?

Answer 2

when the pharmacological effect of two or more drugs given together is not just a direct function of their individual effects

Question 3

what are the drugs that are most commonly involved in drug interactions?

Answer 3

lithium
digoxin
warfarin
theophylline
cyclosporin
phenytoin
gentamicin

Question 4

what increases the chance of a drug-drug interaction occurring?

Answer 4

higher number of medications
being very old or young
critically ill patients
patients undergoing complicated surgical procedures
patients with chronic conditions

Question 5

what are the 3 mechanisms by which drug-drug interactions may occur?

Answer 5

pharmaceutical
pharmacokinetic
pharmacodynamic

Question 6

what are some examples of pharmacodynamic interactions?

Answer 6

antagonistic interactions
additive or synergistic interactions
interactions due to changes in drug transport
interactions due to fluid and electrolyte disturbances
indirect pharmacodynamic interactions

Question 7

what can happen in the GI tract with regards to absorption of drugs?

Answer 7

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

Question 8

what happens in the GI tract with regards to drug binding?

Answer 8

drugs can bind to each other (such as tetracyline and erythromycin complex with iron, calcium and magnesium)

Question 9

why is pH significant in determining the absorption rate of a drug?

Answer 9

absorption is affected by the degree of ionisation which is dependent on pH

Question 10

what can reduce pH?

Answer 10

H2 agonists, proton pump blockers and antacids

Question 11

what is the effect of broad spectrum antibiotics on normal gut flora?

Answer 11

destroys normal gut flora - can lead to OCP failure or digoxin toxicity

Question 12

where are most oral medicines absorbed?

Answer 12

small intestine

Question 13

what is the rate limiting step of absorption of oral medicines?

Answer 13

gastric emptying

Question 14

when does protein binding displacement occur?

Answer 14

when there is a reduction in the extent of plasma protein binding of a drug caused by the presence of another drug

Question 15

what does displacement of a drug from plasma proteins result in/

Answer 15

increased bioavailability of the displaced drug

Question 16

what are the two most important plasma proteins with regard to drug binding?

Answer 16

albumin

alpha 1 glycoprotein

Question 17

when do drug interactions involving metabolism occur?

Answer 17

when one drug induces or inhibits the metabolism of another

Question 18

where does metabolism occur?

Answer 18

commonly in the liver via the cytochrome p450 system

Question 19

what drugs can inhibit the cytochrome system?

Answer 19

clarithromycin
erythromycin
omeprazole
CCBs
these drugs inhibit the metabolism of a small group of drugs metabolised by the cytochrome P450  system

Question 20

what drugs can induce cytochrome P450?

Answer 20

barbiturates
phenytoin - warfarin, steroids
rifampicin - warfarin
tobacco

Question 21

where does drug elimination occur?

Answer 21

the kidney

Question 22

what can inhibit drug excretion?

Answer 22

vrapamil/diltiazem

Question 23

what are the effects of loop diuretics?

Answer 23

increase tubular reabsorption

Question 24

what can beta blockers interact with?

Answer 24

agonists such as salbutamol

Question 25

what are synergistic interactions?

Answer 25

when two drugs with the same pharmacological effect are given concurrently

Question 26

what do NSAIDs interact with?

Answer 26

antihypertensive medication and heart failure treatment