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CLINICALLY IMPORTANT DRUG INTERACTIONS
Dr.R.JAMUNA RANIPROFESSOR & HOD
DEPARTMENT OF PHARMACOLOGY
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DRUG INTERACTION – A REVIEW
Drug interaction refers to modification of response to one drug by another when they are administered simultaneously or in quick succession.
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The modification is mostly quantitative, i.e the response is either increased or decreased in intensity, but sometimes it is qualitative, i.e. an abnormal or a different type of response is produced.
Many medical conditions are treated with a combination of drugs and doctors.
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DESIRABLE DRUG INTERACTION
1. ACE inhibitors + diuretics to treat hypertension
2. Sulfamethoxazole + trimethoprim to treat bacterial infection
3. Furosemide + amiloride to prevent hypokalaemia.
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4. Probenecid + penicillin used in gonococcal infections.
5. Allopurinol + 6-Mercaptopurine & Azathioprine inhibition of metabolism; Reduce dose of 6-MP/azathioprine to 1/3 and used in cancer chemotherapy.
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CLINICALLY IMPORTANT DRUG INTERACTIONS
PRECIPITANT DRUG
OBJECT DRUG LIKELY INTERACTION AND COMMENTS
1. AmpicillinAmoxicillin
Tetracyclines
Oral contraceptives
Interruption of enterohepatic circulation of the estrogen failure of contraception;
Advised alternative contraception
2. AmpicillinAmoxicillin
Tetracyclines
Oral anticoagulants
Inhibition of gut flora decreased vit K production in gut risk of bleeding;
Monitor INR and reduce anticoagulant dose if needed.
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3. CarbenicillinTicarcillin
Aspirin and other antiplatelet drugs
Cause additive platelet inhibition risk of bleeding;
Avoid concurrent use.
4. Ceftriaxone Cefoperazone
Oral anticoagulants Additive hypoprothrombinaemia
bleeding;
Monitor INR and reduce dose of anticoagulant
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5. SulfonamidesCotrimoxazole
Phenytoin Displacement + inhibition of metabolism phenytoin toxicity;
Avoid concurrent use.
6. SulfonamidesCotrimoxazole
Warfarin Displacement + inhibition of metabolism + decreased production of vit K in gut
risk of bleeding;
Monitor INR and reduce dose of warfarin.
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7. SulfonamidesCotrimoxazole
Sulfonylureas Displacement + inhibition of metabolism hypoglycaemia;
Avoid concurrent use.
8. SulfonamidesCotrimoxazole
Thiazide diuretics Increased incidence of thrombocytopenia;
Avoid concurrent use.
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9. MetronidazoleProcarbazineTinidazoleGriseofulvinCefoperazoneCefotetanCeftriaxone
Alcohol Accumulation of acetaldehyde disulfiram – like or bizarre reactions;
Warn the patient not to drink alcohol.
10. Metronidazole Tinidazole
Lithium salts Decreased excretion Li toxicity;
Monitor Li level and reduce lithium dose
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11. Metronidazole Tinidazole
Warfarin Inhibition of metabolism risk of bleeding;
Avoid concurrent use
12. CiprofloxacinNorfloxacinPefloxacin
TheophyllineWarfarin
Inhibition of metabolism toxicity of object drug;
Monitor and reduce dose of object drug.
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13. ErythromycinClarithromycinKetoconazoleItraconazoleFluconazoleProtease inhibitors
TerfenadineAstemizoleCisapride
Inhibition of metabolism by CYP3A4 rise in blood level of object drug dangerous ventricular arrhythmia;
Concurrent use contraindicated.
14. ErythromycinClarithromycinKetoconazoleItraconazoleFluconazoleProtease inhibitors
PhenytoinCarbamazepineWarfarinSulfonylureasDiazepamTheophyllineCyclosporineHIV protease inhibitors
Inhibition of metabolism by CYP3A4 toxicity of object drug;
Avoid concurrent use or readjust dose of object drug.
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15. ErythromycinClarithromycinKetoconazoleItraconazoleFluconazoleProtease inhibitors
Statins Inhibition of metabolism, higher risk of myopathy;
Avoid concurrent use.
16. GemfibrozilNicotinic acid
Statins Increased risk of myopathy;
Caution in concurrent use.
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17. Tetracyclines Lithium salts Rise in plasma Li level due to decreased excretion;
Avoid use of tetracycline or monitor and reduce dose of lithium.
18. Iron saltsCalcium saltsAntacidsSucralfate
TetracyclinesFluoroquinolones
Decreased absorption due to formation of complexes in g.i.t failure of antibiotic therapy;
Stager drug administration by 2-3hours
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19. Furosemide MinocyclineAminoglycosideantibiotics
Enhanced vestibular toxicity; Avoid concurrent use. Additive ototoxicity and nephrotoxicity;
Avoid concurrent use.
20. Diuretics Tetracycline Antianabolic effect of tetracycline increases urea production which is retained by the diuretic;
Avoid concurrent use.
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21. Diuretics Lithium Decreased excretion – rise in Li level – toxicity;
Reduce dose of lithium and monitor level.
22. Diuretics Digoxin Hypokalaemia caused by diuretic increases digoxintoxicity;
Give K+ sparing diuretic /K+ supplements.
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23. TetracyclinesChloramphenicolMacrolide antibioticsClindamycin
PenicillinsCephalosporins
Bactericidal action of penicillins and cephalosporins may be antagonized by the bacteriostatic antibiotics;
Avoid concurrent use.
24. Clindamycin ErythromycinClarithromycinAzithromycinChloramphenicol
Mutual antagonism of antibacterial action due to proximal binding sites on bacterial ribosomes;
Avoid concurrent use.
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25. PhenobarbitonePhenytoinCarbamazepineRifampin
MetronidazoleDoxycyclineChloramphenicolProtease inhibitorsWarfarinCorticosteroidsOral contraceptivesSulfonyl ureasAntidepressants
Induction of metabolism loss of efficacy of object drug;
Avoid concurrent use or increase dose of object drug with monitoring.
26. NSAIDs Ciprofloxacin and Other fluoroquinolones
Enhanced CNS toxicity, seizures;
Avoid concurrent use.
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27. Aspirin and other NSAIDs
Sulfonyl ureasPhenytoinValproateMethotrexate
Displacement and /or reduced elimination toxicity of object drug;
Avoid concurrent use/ substitute NSAID with paracetamol
28. Aspirin and other NSAIDs
WarfarinHeparin
Enhanced risk opf bleeding due to antiplatelet action and gastric mucosal damage;
Avoid concurrent use
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29. Aspirin and other NSAIDs
ACE inhibitorsβ blockersThiazide diuretics
Reduced antihypertensive effect due to inhibition of renal PG synthesis;
Avoid concurrent use
30. Aspirin and other NSAIDs
Furosemide Reduced diuretic action due to PG synthesis inhibition in kidney;
Avoid concurrent use
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31. Allopurinol Ampicillin Increased incidence of rashes;
Avoid concurrent use
32. Chronic alcoholism
Paracetamol Hepatotoxic dose of paracetamol is reduced;
Doses < 3 g/day are safe.
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33.ChlorpromazineHaloperidolMetoclopramide
Levo dopa - carbidopaAntagonism of antiparkinsonian effect;
Concurrent use contraindicated
34. SildenafilTadalafil
Nitrates Marked potentiationprecipitous fall in BP;
Concurrent use contraindicated
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DRUG INTERACTIONS BEFORE ADMINISTRATION
Certain drugs react with each other and get inactivateif their solutions are mixed before administration.
1. Penicillin G or ampicillin mixed with gentamicin or another aminoglycoside antibiotic.
2. Thiopentone sodium when mixed with succinylcholine or morphine.
3. Heparin when mixed with penicillin/ gentamicin/ hydrocortisone.
4. Noradrenaline when added to sodium bicarbonate solution.
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Not all patients taking inter acting drugs experience adverse consequences, However, it is prudent to consider the possibility of drug interaction whenever two or more drugs are prescribed to a patient, or any drug is added to what the patient is already taking.
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