20) Drug interactions – clinical importance Flashcards
(24 cards)
what is Drug interaction
modification in the magnitude or duration of a drug by previous or concomiatant administraion of another
non prescription drugs and food included
effects of drug interaction
desirable
no effect
adverse
desirable interactions:
to enhance the therapeutic efficacy
reduce the risk of adverse effects
drugs affecting different pathogenic mechanisms of the same disease have synergistic effect.
desirable interactions: asthma
Bronchodilators such as beta2-agonistss
combined with potent antinflammatory drugs such as glucocorticosteroids
desirable interactions: antibiotic and ez inhib
combination of amoxicillin and clavulanic acid in co-amoxiclav
clavulanic cid inhib penicillinase increasing spectrum of use
desirable interactions: antibiotic for mixed infev
group 1 and 2
group 1 and 3
group 2 and 2
desirable interactions:
similar drugs cancellin out ADR
thiazide + spironolactone to decrease hypokalemia
desirable interactions: antidote
benzo a.dote= FLUMAZENIL
warfarin adote = vit k
heparin adote = protamini sulfas
Adverse interactions
loss of therapeutic activity
increase of effect to the extent of toxicity
predisposing factors for Adverse interactions
drugs with narrow therapeutic index
enzyme inhibitors or enzyme inducers
impaired excretory organ function of pt
Elderly patients who receive multiple drugs
drugs with high risk for potential adverse interactions
⦁ Anticoagulants ⦁ Anticonvulsants ⦁ Oral hypoglycemic drugs ⦁ Digoxin ⦁ Drugs for HIV
Mechanisms of drug interactions:
Pharmaceutical mechanisms
Pharmacokinetic mechanisms
Pharmacodynamic mechanisms:
examples of Pharmaceutical mechanisms
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
Pharmacokinetic mechanisms:
Interactions affecting drug absorption:
Protein-binding interactions: (distro)
Interactions due to altered biotransformation:(met)
Interactions due to altered renal excretion:
examples of Pharmacokinetic mechanisms: Interactions affecting drug absorption:
⦁ 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
examples of Pharmacokinetic mechanisms Protein-binding interactions
⦁ 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
Interactions due to altered biotransformation
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
examples of cyp450 inhibitors
cimetidine
NON dihydropyradines: verapamil and diltiazem
antimycotics : ketoconaazole
macrolides: clarithromycin
erythromycin
why does ez induction last longer than inhibition
ez inhin only depends on the conc of inhib drug
examples of inducers
tobacco
phenytoin
barbiturates(pheno)
Benzo(carbamazepine)
what happens if the inducing agent is discontinued
primary drug dose must be decreased
Interactions due to altered renal excretion:
Alternation of urine pH
alkaline urine ionises acidic grugs and increasees excreton
example of Pharmacodynamic mechanisms
interactions occur between drugs acting at a common receptor.
β-adrenergic blocking drugs antagonize the effects of β2-adrenergic agonists
salbutamol in asthma
Drug action on body systems
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
