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.
| ANALGESICS |
| 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 |
| |
| ANTACIDS
AND GASTROINTESTINAL |
| 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 |
| Domperidone |
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 |
| |
| COUGH,
COLDS, SORE THROAT AND HAYFEVER |
| 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
& ß 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 |
| ß
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 ß-blockers |
Bronchospasm |
| |
| SKIN
CARE AND ANTISEPTICS |
| 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 |
| |
| VITAMINS,
MINERALS AND OTHER NUTRITIONAL |
| 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 |
| |
| ANTIMALARIALS |
| OTC
Medicine or Class |
Interacting
Medicine or Class |
Nature
of Interaction |
| Chloroquine |
Ampicillin
Phenothiazine |
Reduced
absorption
Increased serum levels |