2017 HCHB_digital
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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