20) Drug interactions – clinical importance

Question 1

what is Drug interaction

Answer 1

modification in the magnitude or duration of a drug by previous or concomiatant administraion of another
non prescription drugs and food included

Question 2

effects of drug interaction

Answer 2

desirable

no effect

adverse

Question 3

desirable interactions:

Answer 3

to enhance the therapeutic efficacy

reduce the risk of adverse effects

drugs affecting different pathogenic mechanisms of the same disease have synergistic effect.

Question 4

desirable interactions: asthma

Answer 4

Bronchodilators such as beta2-agonistss

combined with potent antinflammatory drugs such as glucocorticosteroids

Question 5

desirable interactions: antibiotic and ez inhib

Answer 5

combination of amoxicillin and clavulanic acid in co-amoxiclav

clavulanic cid inhib penicillinase increasing spectrum of use

Question 6

desirable interactions: antibiotic for mixed infev

Answer 6

group 1 and 2

group 1 and 3

group 2 and 2

Question 7

desirable interactions:

similar drugs cancellin out ADR

Answer 7

thiazide + spironolactone to decrease hypokalemia

Question 8

desirable interactions: antidote

Answer 8

benzo a.dote= FLUMAZENIL

warfarin adote = vit k

heparin adote = protamini sulfas

Question 9

Adverse interactions

Answer 9

loss of therapeutic activity

increase of effect to the extent of toxicity

Question 10

predisposing factors for Adverse interactions

Answer 10

drugs with narrow therapeutic index

enzyme inhibitors or enzyme inducers

impaired excretory organ function of pt

Elderly patients who receive multiple drugs

Question 11

drugs with high risk for potential adverse interactions

Answer 11

⦁	Anticoagulants
⦁	Anticonvulsants 
⦁	Oral hypoglycemic drugs
⦁	Digoxin
⦁	Drugs for HIV

Question 12

Mechanisms of drug interactions:

Answer 12

Pharmaceutical mechanisms

Pharmacokinetic mechanisms

Pharmacodynamic mechanisms:

Question 13

examples of Pharmaceutical mechanisms

Answer 13

mixing two drug solutions in a syringe or in iv infusion can lead to

1) precipitation
e. g cephalosporins and calcium salts

2) decreased activity of a drug
- e.g hydrocortisone inactivating heparin

Question 14

Pharmacokinetic mechanisms:

Answer 14

Interactions affecting drug absorption:

Protein-binding interactions: (distro)

Interactions due to altered biotransformation:(met)

Interactions due to altered renal excretion:

Question 15

examples of Pharmacokinetic mechanisms: Interactions affecting drug absorption:

Answer 15

⦁ Direct chemical interaction: Antacids form insoluble complexes with oral tetracyclines and reduce their absorption.

Alteration of gastric emptying and gut motility:
Drugs the increase or reduce the rate of gastric emptying can alter the rate of absorption of other drugs.

anti/cholinergics

Question 16

examples of Pharmacokinetic mechanisms Protein-binding interactions

Answer 16

⦁ Drugs that are extensively bound to plasma proteins are potentially subject to displacement from these proteins by other drugs with high affinity for the same protein. This increases the free concentration of the displaced drug that may contribute to an adverse reaction.

e.g displacement of warfarin by rifampicin

Question 17

Interactions due to altered biotransformation

Answer 17

drugs can induce or imhibit heoatc cytochrome ez

ez inhibition decreases met and increases risk of adr

ez induction increases met and decreases effect of drug

Question 18

examples of cyp450 inhibitors

Answer 18

cimetidine

NON dihydropyradines: verapamil and diltiazem

antimycotics : ketoconaazole

macrolides: clarithromycin
erythromycin

Question 19

why does ez induction last longer than inhibition

Answer 19

ez inhin only depends on the conc of inhib drug

Question 20

examples of inducers

Answer 20

tobacco
phenytoin
barbiturates(pheno)
Benzo(carbamazepine)

Question 21

what happens if the inducing agent is discontinued

Answer 21

primary drug dose must be decreased

Question 22

Interactions due to altered renal excretion:

Answer 22

Alternation of urine pH

alkaline urine ionises acidic grugs and increasees excreton

Question 23

example of Pharmacodynamic mechanisms

Answer 23

interactions occur between drugs acting at a common receptor.

β-adrenergic blocking drugs antagonize the effects of β2-adrenergic agonists

salbutamol in asthma

Question 24

Drug action on body systems

Answer 24

heart

• ve inotroopic drugs shouldn.t be used
• verapamil x betablocker

arrythmic effect of digoxin is increased by thiazide diuretics d/2 hypokalemia

CI of NSAIDS x antihypertensives d/2 pg inhib causing Na+ retention