The following drug interactions are 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.
Drug interactions
ANALGESICS
OTC MEDICINE OR CLASS
INTERACTING MEDICINE OR CLASS
NATURE OF INTERACTION
| Aspirin, Salicylates |
Anticoagulants, antiplatelets and thrombolytics |
Increased risk of bleeding due to antiplatelet effect |
| Carbonic anhydrase inhibitors (acetazolamide) |
Increased plasma levels of acetazolamide |
| Cytotoxics (methotrexate) |
Reduced excretion of methotrexate |
| NSAIDs, corticosteroids |
Increased likelihood of stomach irritation and gastro-intestinal bleeding |
| Uricosurics (sulfinpyrazone) |
Reduced uricosuric effects |
| Doxylamine |
Monoamine oxidase inhibitors |
Increased risk of antimuscarinic side-effects |
| Hyoscine butylbromide |
Tricylic antidepressants, antipsychotics, antihistamines, quinidine, anticholinergics (eg ipratropium, tiotropium) |
Intensifies antimuscarinic effect |
| NSAIDs (ibuprofen, naproxen) |
ACE inhibitors, angiotensin II inhibitors, potassium salts, potassium-sparing diuretics, trimethoprim |
Increased risk of hyperkalaemia and renal impairment; increased blood pressure |
| Antibacterials (quinolones) |
Increased risk of convulsions |
| Antihypertensives, diuretics |
Reduced diuretic and antihypertensive effect |
| Other NSAIDs (including aspirin), corticosteroids, anticoagulants |
Increased likelihood of stomach irritation or bleeding (NSAIDs, anticoagulants), reduced antiplatelet effects (aspirin) |
| Methotrexate |
Increased toxicity |
| Lithium |
Reduced excretion |
| Tacrolimus |
Increased risk of nephrotoxicity |
| Paracetamol |
Alcohol |
Increased susceptibility to liver damage in heavy drinkers |
| Colestyramine |
Reduced absorption of paracetamol |
| Antiemetics (metoclopramide, domperidone) |
Increased rate of absorption of paracetamol |
| Codeine |
Antidepressants, antihistamines, alcohol, antipsychotics, anxiolytics, hypnotics, concurrent opioid analgesics |
Increased sedation |
ANTACIDS AND GASTROINTESTINAL
OTC MEDICINE OR CLASS
INTERACTING MEDICINE OR CLASS
NATURE OF INTERACTION
| Antacids, Adsorbents |
Ketoconazole, chloroquine, penicillamine, mycophenolate |
Reduced absorption of these drugs |
| Analgesics (aspirin) |
Reduced serum salicylate levels and reduced absorption |
| Some quinolone antibiotics (eg ciprofloxacin,rifampicin, tetracyclines, azithromycin, erythromycin) |
May interfere with absorption or peak levels |
| Bisphosphonates |
Reduced absorption of bisphosphonates |
| Proguanil |
Absorption reduced by magnesium trisilicate |
| Thiazide diuretics |
Increased risk of hypercalcaemia |
| Tetracycline antibiotics (eg doxycycline) |
Interferes with absorption of the antibiotic |
| Mebendazole |
Carbamazepine, phenytoin |
Lowers plasma mebendazole |
| Metronidazole |
Increased risk of Stevens-Johnson syndrome |
| Omeprazole, Esomeprazole |
Antifungals (itraconazole, ketoconazole) |
Reduced absorption of these drugs |
| Anxiolytic (diazepam) |
Enhanced effect of diazepam |
| Cilostazol |
Increased plasma levels of cilostazol |
| Clopidogrel |
Reduced efficacy of clopidogrel |
| Voriconazole |
Increased plasma levels of omeprazole/esomeprazole |
| Ranitidine |
Ketoconazole, atazanavir |
Increased absorption of these drugs |
ANTIMALARIALS
OTC MEDICINE OR CLASS
INTERACTING MEDICINE OR CLASS
NATURE OF INTERACTION
| Antimalarials (atovaquone) |
Tetracycline antibiotics (eg doxycycline) |
Decreased plasma levels of atovaquone |
| Metoclopramide |
Decreased plasma levels of atovaquone |
COUGH, COLDS, SORE THROAT / HAYFEVER AND OTHER ALLERGIES
OTC MEDICINE OR CLASS
INTERACTING MEDICINE OR CLASS
NATURE OF INTERACTION
| Loratadine |
MAOIs (phenelzine, tranylcypromine) |
Increased risk of antimuscarinic adverse effects |
| Sedative antihistamines (eg chlorphenamine, diphenhydramine) |
Promethazine, tricyclic antidepressants |
Enhances anticholinergic action |
| MAOIs |
Intensifies anticholinergic properties |
| Alcohol, benzodiazepines and other sedatives, tricyclic antidepressants |
Increased risk of CNS depressant effects |
| Sympathomimetics (eg ephedrine, pseudoephedrine, phenylephrine, promethazine, triprolidine, xylometazoline) |
MAOIs |
Increased risk of hypertensive crisis |
| Other sympathomimetic agents (decongestants, tricyclic antidepressants), bromocriptine |
Potentiates effect |
EYE CARE
OTC MEDICINE OR CLASS
INTERACTING MEDICINE OR CLASS
NATURE OF INTERACTION
| Chloramphenicol (topical) |
Bone marrow depressant drugs |
Increased bone marrow depression |
FEMALE HEALTH AND GENITOURINARY
OTC MEDICINE OR CLASS
INTERACTING MEDICINE OR CLASS
NATURE OF INTERACTION
| Desogestrel and Levonorgestrel |
CYP3A4 inducers (eg rifampicin, phenytoin, carbamazepine, St John’s Wort) |
Reduced efficacy of levonorgestrel |
| Tranexamic acid |
Fibrinolytics (eg streptokinase) |
Counteracts thrombolytic effect of fibrinolytics |
| Ulipristal acetate |
CYP3A4 inducers (eg rifampicin, phenytoin, carbamazepine, St John’s Wort) |
Reduced efficacy of ulipristal |
| Hormonal contraceptives |
May reduce contraceptive action of combined hormonal and progesterone-only contraceptives |
HAIR AND SCALP
OTC MEDICINE OR CLASS
INTERACTING MEDICINE OR CLASS
NATURE OF INTERACTION
| Topical minoxidil |
Other medications applied topically to the scalp (eg corticosteroids, tretinoin, dithranol) |
Increased absorption of minoxidil |
MALE HEALTH
OTC MEDICINE OR CLASS
INTERACTING MEDICINE OR CLASS
NATURE OF INTERACTION
| Sildenafil |
Nicorandil |
Increased risk of hypotension |
| Nitrates (eg glyceryl trinitrate, isosorbide mononitrate, isosorbide dinitrate) |
Increased risk of hypotension |
| CYP3A4 inhibitors (eg ketoconazole, diltiazem, erythromycin, rifampicin) |
Increased exposure to sildenafil |
| Antihypertensives |
Increased risk of hypotension |
| Tamsulosin |
Ketoconazole |
Increased plasma levels of tamsulosin |
| Antihypertensives |
Increased risk of hypotension |
MIGRAINE RELIEF
OTC MEDICINE OR CLASS
INTERACTING MEDICINE OR CLASS
NATURE OF INTERACTION
| Sumatriptan |
MAOIs, SSRIs, tricyclic antidepressants, St John’s Wort, fentanyl, pethidine, tramadol |
Increased risk of serotonin syndrome |
| Ergot alkaloids |
Increased risk of vasoconstriction |
| Prochlorperazine maleate |
Antihypertensives (eg ramipril, losartan, sotalol) |
Increased risk of hypotension |
| Lithium |
Risk of severe neurotoxicity |
| Alcohol |
Increased risk of hypotension |
| CNS depressants (eg tramadol, aripiprazole, chlorphenamine) |
Additive CNS depressant effect |
SKIN CARE AND ANTISEPTICS
OTC MEDICINE OR CLASS
INTERACTING MEDICINE OR CLASS
NATURE OF INTERACTION
| Clotrimazole (cream) |
Latex contraceptives |
Reduced effectiveness of latex contraceptives |
| Tacrolimus |
Increased plasma levels of tacrolimus |
| Oral fluconazole |
Warfarin, phenytoin, simvastatin, midazolam, sulphonylureas |
Decreased metabolism of these drugs |
| Bendroflumethiazide, furosemide, ritonavir, zidovudine |
May lead to hypokalaemia |
| Rifampicin, phenytoin |
Increased metabolism of fluconazole |
| Warfarin |
Increased anticoagulant effect |
SMOKING CESSATION
OTC MEDICINE OR CLASS
INTERACTING MEDICINE OR CLASS
NATURE OF INTERACTION
| Nicotine |
Anti-arrythmics (adenosine) |
Action potentiated |
VITAMINS, MINERALS AND NUTRITIONALS
OTC MEDICINE OR CLASS
INTERACTING MEDICINE OR CLASS
NATURE OF INTERACTION
| Vitamin A |
Retinoids |
Increased risk of vitamin A toxicity |
| Vitamin B3 (niacin) |
Glimepiride |
May increase blood sugar levels; may increase requirements for the drug |
| Vitamin B6 (pyridoxine) |
Phenobarbital, phenytoin |
May reduce drug absorption and effectiveness |
| Vitamin C |
Aspirin |
Reduced vitamin C absorption |
| Vitamin D |
Antiepileptics (phenytoin) |
Reduced serum vitamin D levels |
| Thiazide diuretics |
Increased risk of hypercalcaemia |
| Vitamin E |
Aspirin |
May increase risk of bleeding |
| Vitamin K |
Anticoagulants (eg warfarin) |
Reduced anticoagulant effect |
| Agnus Castus |
Dopamine-receptor antagonists |
May reduce effectiveness |
| Dopamine-receptor agonists |
Possible potentiation of effect |
| Calcium |
Thiazide diuretics |
Increased risk of hypercalcaemia |
| Antibacterials (ciprofloxacin, tetracyclines, quinolones), bisphosphonates, nadolol, beta-blockers |
May reduce drug absorption |
| Chromium |
Anti-diabetic medications (insulin) |
Increased risk of hypoglycaemia |
| Co-enzyme Q-10 |
Warfarin |
Reduced anticoagulant effect |
| Copper |
Ciprofloxacin |
May reduce drug absorption |
| Cranberry |
Warfarin |
May inhibit breakdown of warfarin in the body, leading to increased warfarin activity |
| Echinacea |
Ciclosporin |
May reduce efficacy |
| Methotrexate |
May reduce efficacy |
| Feverfew |
Anticoagulants, antiplatelets, NSAIDs |
May increase risk of bleeding |
| Fish oil/cod liver oil (EPA & DHA) |
Antihypertensives |
May reduce blood pressure |
| Anticoagulants, antiplatelets |
May increase anti-clotting effects, leading to bleeding |
| Folic acid |
Antiepileptics (phenobarbital, phenytoin) |
May decrease levels of some antiepileptics |
| Methotrexate |
Reduced plasma levels of folic acid |
| Garlic |
Anticoagulants, antiplatelets |
May increase anti-clotting effects, leading to bleeding |
| Ginger |
Anticoagulants (eg warfarin) |
May increase anti-clotting effects, leading to bleeding |
| Glucosamine |
Anticoagulants (eg warfarin) |
Enhanced anticoagulant effect |
| Iodine |
Antithyroids |
Increased risk of hypothyroidism |
| Iron |
Magnesium trisilicate |
Reduced absorption of oral iron |
| Dopaminergics (levodopa), penicillamine, quinolone antibiotics (eg ciprofloxacin, norfloxacin, ofloxacin), zinc, bisphosphonates, levothyroxine, warfarin |
Reduced absorption of these drugs |
| Tetracycline antibiotics |
Reduced absorption of iron and these drugs |
| Magnesium |
Quinolone antibiotics (eg ciprofloxacin, norflaxacin, ofloxacin), tetracycline antibiotics, bisphosphonates |
May decrease drug absorption |
| Misoprostol |
Diarrhoea |
| Panax Ginseng |
Imatinib |
May increase toxicity of drug |
| Anticoagulants (eg warfarin) |
May reduce anticoagulant effect |
| Passion flower |
Sedatives (eg benzodiazepines) |
May increase sedative effect |
| Pelargonium |
Anticoagulants (eg warfarin) |
May increase anticoagulant effect |
| Potassium |
ACE inhibitors, beta-blockers, ciclosporin, potassium sparing diuretics, tacrolimus, co-trimoxazole |
Increased risk of hyperkalaemia |
| Red clover |
Oestrogens |
May decrease the effect of oestrogens |
| Anticoagulants (eg warfarin) |
May increase risk of bleeding |
| St John’s Wort |
Interacts with a number of medications. A full list can be found at: https://bnf.nice.org.uk/interaction/st-johns-wort-2.html |
St John’s Wort decreases the plasma levels of several medicines. Some interactions are severe |
| Valerian |
Hypnotics and other sedatives |
May increase sedative effect |
| Zinc |
Tetracycline antibiotics, quinolone antibiotics (ciprofloxacin, penicillamine) |
Reduced absorption of these drugs |
WEIGHT MANAGEMENT
OTC MEDICINE OR CLASS
INTERACTING MEDICINE OR CLASS
NATURE OF INTERACTION
| Orlistat |
Antiepileptics |
May increase risk of convulsions |
| Ciclosporin |
May reduce absorption of ciclosporin |
| Oral contraceptives |
Decreased effect of oral contraceptives |
| Fat soluble vitamins (A, D, E, K) |
Decreased absorption of the fat-soluble vitamins |
