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Drug Interactions

The following are drug interactions considered pharmacologically significant although many will be of little clinical significance in short term OTC use. All but a few occur with medicines available only on prescription.

OTC Medicine or Class Interacting Medicine or Class Nature of Interaction
Aspirin and salicylates Antacids Reduction in serum salicylate levels
Anticoagulants (heparin, warfarin) Increased risk of bleeding due to antiplatelet effect
Antiepileptics (phenytoin, valproate)
Carbonic anhydrase inhibitors (acetazolamide, dichlorphenamide) Increased serum level of phenytonin, valproate
Increased risk of salicylate toxicity
Cytotoxics (methotrexate) Reduced excretion of methotrexate
Diuretics Reduced diuretic effect
NSAIDs and corticosteroids Increased likelihood of stomach irritation
Sulphonylureas Increases effects
Uricosurics (probenecid, sulphinpyrazone) Reduced uricosuric effects
Ibuprofen Diuretics Reduced diuretic and antihypertensive effect
NSAIDs and corticosteroids Increases likelihood of stomach irritation
Paracetamol Alcohol Increased susceptibility to liver damage in heavy drinkers
Anion exchange resins (cholestyramine) Reduces absorbsion of paracetamol
Metoclopramide, domperidone Increases onset of action of paracetamol
Codeine and Hyoscine Antidepressants, antihistamines, antipsychotics, anxiolytics & hypnotics Increases sedation
Quinine Digitalis glycosides Plasma digoxin increased
Cimetidine Metabolism of quinine inhibited
OTC Medicine or Class Interacting Medicine or Class Nature of Interaction
Antacids and Adsorbents Some ACE-inhibitors, antifungals (itraconazole, ketoconazole), antimalarials (chloroquine), anxiolytics, corticosteroids & penicillamine Reduced absorption of these drugs
Analgesics (asprin, diflunisal) Reduced serum salicylate levels & reduced absorption of diflunisal
Antiarrhythmics (quinidine) Reduced excretion of quinidine
Antibacterials (ciprofloxacin, enoxacin, norfloxacin, perfloxacin, rifampicin, tetracycline) Reduced absorption of antibacterials
Anticoagulants (dicoumarol, warfarin) Not affected by aluminium hydroxide. Magnesium hydroxide & trisilicate increase absorption
Antiepileptics (phenytoin) & Antipsychotics (phenothiazines) Reduced absorption of phenytoin and phenothiazines
Enteric coated tablets (bisacodyl) Breaks up enteric coating
Iron Absorption of iron reduced by magnesium trisilicate
Cimetidine Oral anticoagulants (nicoumalone/acenocoumarol warfarin) Metabolism of anticoagulant inhibited
Antiepileptics, (carbazepine, phenytoin) Metabolism of aniepileptic inhibited
Theophyllline Metabolism of theophylline inhibited
Lignocaine/lidocaine (intravenous) Metabolism of lignocaine/lidocaine inhibited
Chloroquine Reduced metabolism
Sodium salts Lithium Reduced lithium levels
Mebendazole Carbamazepine & phenytoin Lowers serum mebendazole
Cimetidine Raises serum mebendazole levels increasing its effectiveness

Dopamine agonists Antimuscarinics and opioid analgesics

Concurrently administered medicines

May alter peripheral action Possible reduced effectiveness of domperidone Possible enhanced absorption because of delayed gastric emptying
OTC Medicine or Class Interacting Medicine or Class Nature of Interaction
Sedative antihistamines (eg azatadine, bromopheniramine, carbinoxamine, chloropheniramine, diphenhydramine, doxylamin, pheniramine, promethazine, triprolidine) Alcohol, anticholinergics (benzhexol, orphenadrine), benzodiazepines and other sedatives, tricyclic antidepressants Additive effect to cause increased sedation or antimuscarinic effects
Sympathomimetics (eg ephedrine, pseudoephedrine, phenylephrine, phenylpropanolamine) Digitalis Increased risk of arrhythmias
Hypotensive agents which interfere with sympathetic activity (a & adrenergic blockers, bethanidine, bretylium, debrisoquine, guanethidine, methyldopa) Reduced action of these drugs leads to increased blood pressure
Mono-amine oxidase inhibitors Hypertensive crisis
Other sympathomimetic agents (decongestants, tricyclic antidepressants) Potentiates effect
receptor stimulants or xanthine derivatives (theobromine, theophylline) Potentiates effect
Theophylline Allopurionol, antibacterials (ciprofloxacin, enoxacin, erythoromycin, norfloxacin, triacetyloleandomycin), calcium channel blockers (diltiazem, verapamil), cimetidine, disulfiram, fluconazole, idrocilamide, influenza vaccine, interferon, combined oral contraceptives, viloxazine, thiabendazole Increased plasma theophylline levels
Aminogluetethimide, antiepileptics, (carbamazepine, phenobarbitone, primidone, phenytoin), barbituates and rifampicin Reduced in plasma theophylline levels
Anaesthetics (halothane) Increased risk of arrhythmias
Bronchodilators (salbutamol, terbutaline) Increased risk of hypokalaemia if given with high doses of these drugs
Lithium Lithium levels reduced
Non selective -blockers Bronchospasm
OTC Medicine or Class Interacting Medicine or Class Nature of Interaction
Oral Fluconazole Warfarin, sulphonylureas, phenytoiin, astermizole, terfenadine, theophylline Decreased metabolism of these drugs
Hydrochlorothiazide Decreased metabolism of fluconazole
Rifampicin Increased metabolism of fluconazole
OTC Medicine or Class Interacting Medicine or Class Nature of Interaction
Vitamin C Aspirin Vitamin C absorption is reduced
Vitamin D Phenytoin Reduced serum vitamin D levels
Vitamin K Anticoagulants (nicoumaline/acenocoumarol, phenindione, warfarin) Effects of anticoagulants reduced
Calcium Diuretics Increased risk of hypercalcaemia
Folic Acid Anticonvulsants (phenobarbitone, phenytoin, primidone) Reduced serum levels of these drugs
Iodine Diuretics Increased risk of hypothyroidism
Iron Magnesium trisilicate Absorption of oral iron reduced
Dopaminergics (levodopa), penicillamine, quinolone antibiotics (ciprofloxacin, norfloxacin, ofloxacine) Absorption of these drugs reduced
Tetracycline antibiotics and zinc Reduced absorption of oral iron (and visa versa)
Potassium supplements ACE-inhibitors, cyclosporin & potassium sparing diuretics Hyperlkalaemia
Zinc Iron & Tetracycline antibiotics Reduced absorption of zinc (and visa versa)
Quinolone antibiotics (ciprofloxacin) Reduced absorption of ciprofloxacin
OTC Medicine or Class Interacting Medicine or Class Nature of Interaction
Chloroquine Ampicillin
Reduced absorption
Increased serum levels
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Copyright PAGB (Proprietary Association of Great Britain) - 2003 - All Rights Reserved. PAGB has made every effort to ensure the information contained in this site is as accurate and up to date as possible. However you should always read the label before you take a medicine for the first time as it is important that medicines are used correctly and according to the instructions. All OTC medicines are effective and safe when used properly. None of the information given in this directory should be construed to be advice or recommendation of a particular medicine. PAGB cannot accept responsibility for errors or omissions.