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OTC Medicines: Interactions<br />

OTC Medicines: Interactions<br />

Drug/drug group Interacting substance Details<br />

Iron supplements (oral) Levothyroxine Poorly soluble chelate formed which reduces absorption of levothyroxine<br />

Separate administration by at least 2–3 hours. Monitor patient for reduced response<br />

Methyldopa<br />

Reduced absorption of methyldopa may reduce antihypertensive effect. Separate administration by at least<br />

2–3 hours<br />

Penicillamine<br />

Reduced absorption of penicillamine. Separate administration by at least 2–3 hours<br />

Vitamin E<br />

May reduce absorption of vitamin E. Separate administration by at least 2–3 hours<br />

Zinc<br />

Reduced absorption of iron and zinc. Separate administration by at least 2–3 hours<br />

Laxatives<br />

– bulk laxatives<br />

Levonorgestrol<br />

(emergency and oral<br />

contraceptive pill)<br />

Digoxin<br />

Lithium<br />

Enzyme inducers (eg, barbiturates,<br />

carbamazepine, primidone, phenytoin,<br />

topiramate, St John’s wort (Hypericum<br />

perforatum), rifampicin, ritonavir,<br />

griseofulvin)<br />

May decrease digoxin absorption resulting in decreased digoxin levels<br />

Reports of reduced lithium absorption resulting in reduced plasma levels. Monitor lithium levels, separate<br />

administration by at least 2 hours, or use alternative laxative<br />

Increased hepatic metabolism of levonorgestrol and possible reduction in effectiveness if used as<br />

emergency contraception. Consider increasing dose of levonorgestrol<br />

Meningococcal vaccine Immunosuppressive therapies May reduce the immune response to the vaccine<br />

Metoclopramide Alcohol Additive CNS effects (eg, sedation)<br />

Aspirin, paracetamol<br />

Increased rate of aspirin and paracetamol absorption. Combination may be beneficial for migraine attacks<br />

Bromocriptine<br />

Antagonises hypoprolactinaemic effect of bromocriptine<br />

Cyclosporin<br />

Increased absorption of cyclosporin resulting in increased plasma levels. Monitor<br />

Codeine, other opioid analgesics Possible reduction in prokinetic effect of metoclopramide<br />

Dantrolene<br />

Increased absorption of dantrolene resulting in increased plasma levels. Monitor for signs of dantrolene<br />

toxicity (eg, CNS disturbances, diarrhoea)<br />

Digoxin<br />

Absorption of digoxin decreased possibly by increased gut motility. Plasma levels of digoxin may be<br />

reduced. Monitor<br />

Lithium<br />

Risk of extrapyramidal adverse effects or severe neurotoxicity increased.<br />

Avoid combination if possible or carefully monitor for signs of neurotoxicity<br />

MAOIs (irreversible eg, phenelzine, Possible additive hypertensive effects. Avoid combination if possible, or monitor<br />

tranylcypromine)<br />

Medicines with antidopaminergic<br />

actions (eg, antipsychotics,<br />

tetrabenazine)<br />

Increased risk of extrapyramidal adverse effects<br />

Parkinson’s drugs (eg, cabergoline,<br />

levodopa, lisuride)<br />

Medications with antimuscarinic<br />

effects (eg, amantadine, benztropine,<br />

bromocriptine, levodopa, selegiline,<br />

pergolide, procyclidine, sedating<br />

antihistamines, phenothiazines, tricyclic<br />

antidepressants, orphenadrine)<br />

Morphine<br />

SSRIs<br />

Metoclopramide is a dopamine antagonist and antagonises action of medications used to treat Parkinson’s<br />

disease. Avoid combination<br />

Possible reduction in prokinetic effect of metoclopramide<br />

Additive CNS effects (eg, sedation) possible. Increased rate of morphine absorption (faster onset of action)<br />

Possible increase in metoclopramide absorption with increased risk of adverse effects (eg,extrapyramidal<br />

effects)<br />

Methyl salicylate<br />

Warfarin<br />

Topical use of methyl salicylate has been shown to increase INR/PT and result in bleeding and bruising.<br />

(topical)<br />

Present in many OTC analgesic liniments, creams or medicated oils. Use alternative topical analgesics if<br />

possible, or use with great caution and monitoring<br />

Nasal corticosteroids Other corticosteroid use, eg, inhaled Additive systemic effects of corticosteroids<br />

Omeprazole,<br />

Antifungals<br />

Reduces absorption of itraconazole and ketoconazole<br />

lansoprazole<br />

Antiretroviral medications (eg,<br />

atazanavir, nelfinavir, saquinavir)<br />

Clopidogrel<br />

Clozapine<br />

Digoxin<br />

Medications metabolised by CYP 2C19<br />

(eg, citalopram, diazepam, phenytoin,<br />

warfarin, other vitamin K antagonists)<br />

Other CYP 2C19 or CYP 3A4 inhibitors<br />

(eg, clarithromycin, voriconazole)<br />

Decreases serum levels of atazanavir, nelfinavir (avoid combination). Increases levels of saquinavir (contact<br />

prescriber)<br />

Reduces efficacy of clopidogrel (approximately 30% less inhibition of platelet aggregation)<br />

Three reports of elevated clozapine levels documented (two resulted in seizures). Monitor<br />

Reduces absorption of digoxin. Monitor<br />

Theoretically may inhibit the metabolism of these agents, but few clinical reports documented. Monitor<br />

May increase serum levels of omeprazole or lansoprazole, although dosage adjustment not usually<br />

required<br />

Page 188 HEALTHCARE HANDBOOK <strong>2017</strong>-2018 References Charts

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