10 months ago

2017 HCHB_digital

OTC Medicines:

OTC Medicines: Interactions OTC Medicines: Interactions Drug/drug group Interacting substance Details Iron supplements (oral) Levothyroxine Poorly soluble chelate formed which reduces absorption of levothyroxine Separate administration by at least 2–3 hours. Monitor patient for reduced response Methyldopa Reduced absorption of methyldopa may reduce antihypertensive effect. Separate administration by at least 2–3 hours Penicillamine Reduced absorption of penicillamine. Separate administration by at least 2–3 hours Vitamin E May reduce absorption of vitamin E. Separate administration by at least 2–3 hours Zinc Reduced absorption of iron and zinc. Separate administration by at least 2–3 hours Laxatives – bulk laxatives Levonorgestrol (emergency and oral contraceptive pill) Digoxin Lithium Enzyme inducers (eg, barbiturates, carbamazepine, primidone, phenytoin, topiramate, St John’s wort (Hypericum perforatum), rifampicin, ritonavir, griseofulvin) May decrease digoxin absorption resulting in decreased digoxin levels Reports of reduced lithium absorption resulting in reduced plasma levels. Monitor lithium levels, separate administration by at least 2 hours, or use alternative laxative Increased hepatic metabolism of levonorgestrol and possible reduction in effectiveness if used as emergency contraception. Consider increasing dose of levonorgestrol Meningococcal vaccine Immunosuppressive therapies May reduce the immune response to the vaccine Metoclopramide Alcohol Additive CNS effects (eg, sedation) Aspirin, paracetamol Increased rate of aspirin and paracetamol absorption. Combination may be beneficial for migraine attacks Bromocriptine Antagonises hypoprolactinaemic effect of bromocriptine Cyclosporin Increased absorption of cyclosporin resulting in increased plasma levels. Monitor Codeine, other opioid analgesics Possible reduction in prokinetic effect of metoclopramide Dantrolene Increased absorption of dantrolene resulting in increased plasma levels. Monitor for signs of dantrolene toxicity (eg, CNS disturbances, diarrhoea) Digoxin Absorption of digoxin decreased possibly by increased gut motility. Plasma levels of digoxin may be reduced. Monitor Lithium Risk of extrapyramidal adverse effects or severe neurotoxicity increased. Avoid combination if possible or carefully monitor for signs of neurotoxicity MAOIs (irreversible eg, phenelzine, Possible additive hypertensive effects. Avoid combination if possible, or monitor tranylcypromine) Medicines with antidopaminergic actions (eg, antipsychotics, tetrabenazine) Increased risk of extrapyramidal adverse effects Parkinson’s drugs (eg, cabergoline, levodopa, lisuride) Medications with antimuscarinic effects (eg, amantadine, benztropine, bromocriptine, levodopa, selegiline, pergolide, procyclidine, sedating antihistamines, phenothiazines, tricyclic antidepressants, orphenadrine) Morphine SSRIs Metoclopramide is a dopamine antagonist and antagonises action of medications used to treat Parkinson’s disease. Avoid combination Possible reduction in prokinetic effect of metoclopramide Additive CNS effects (eg, sedation) possible. Increased rate of morphine absorption (faster onset of action) Possible increase in metoclopramide absorption with increased risk of adverse effects (eg,extrapyramidal effects) Methyl salicylate Warfarin Topical use of methyl salicylate has been shown to increase INR/PT and result in bleeding and bruising. (topical) Present in many OTC analgesic liniments, creams or medicated oils. Use alternative topical analgesics if possible, or use with great caution and monitoring Nasal corticosteroids Other corticosteroid use, eg, inhaled Additive systemic effects of corticosteroids Omeprazole, Antifungals Reduces absorption of itraconazole and ketoconazole lansoprazole Antiretroviral medications (eg, atazanavir, nelfinavir, saquinavir) Clopidogrel Clozapine Digoxin Medications metabolised by CYP 2C19 (eg, citalopram, diazepam, phenytoin, warfarin, other vitamin K antagonists) Other CYP 2C19 or CYP 3A4 inhibitors (eg, clarithromycin, voriconazole) Decreases serum levels of atazanavir, nelfinavir (avoid combination). Increases levels of saquinavir (contact prescriber) Reduces efficacy of clopidogrel (approximately 30% less inhibition of platelet aggregation) Three reports of elevated clozapine levels documented (two resulted in seizures). Monitor Reduces absorption of digoxin. Monitor Theoretically may inhibit the metabolism of these agents, but few clinical reports documented. Monitor May increase serum levels of omeprazole or lansoprazole, although dosage adjustment not usually required Page 188 HEALTHCARE HANDBOOK 2017-2018 References Charts

OTC Medicines: Interactions Drug/drug group Interacting substance Details Oral contraceptives Antibiotics or other medicines (eg, orlistat) that cause diarrhoea or vomiting Enzyme inducers (eg, aprepitant, bosentan, carbamazepine, efavirenz, modafinil, nelfinavir, nevirapine, oxcarbazepine, perampanel, phenobarbital, phenytoin, primidone, rifabutin, rufinamide, rifampicin, ritonavir, St John’s Wort, topiramate) May reduce the absorption of oral contraceptives. However, generally antibiotics that are not enzymeinducing do not interact with oral contraceptives Increase the metabolism of combinined oral contraceptives and may reduce ovulation suppression Orlistat Amiodarone Decreased absorption of amiodarone. Plasma levels of amiodarone may be reduced. Monitor Cyclosporin Markedly decreased cyclosporin levels. Avoid combination Povidone iodine (see also iodine supplements [oral]) Fat-soluble vitamins (eg, vitamin A, D, E, K) Oral contraceptives Warfarin Lithium Decreased absorption of fat-soluble vitamins. Separate administration by at least 2 hours or take vitamins at bed time Absorption of oral contraceptives may be compromised by any diarrhoea caused by orlistat. Possible failure of contraceptive action Isolated report of INR increase, possibly due to reduction in vitamin K absorption. Monitor INR Additive hypothyroid effect possible with long term topical povidone iodine administration Prochlorperazine Antihypertensives Possible additive hypotensive effect CNS depressants (including alcohol, Additive CNS depressant effects (eg, sedation) opiods) Drugs causing QT prolongation (eg, Possible increased risk QT prolongation (rare reports) erythromycin, haloperidol, sotalol) Lithium Possible increased risk of neurological adverse effects Other agents with anticholinergic/ Additive cholinergic adverse effects (eg, dry mouth, urinary retention, constipation, confusion in the elderly) antimuscarinic effects (eg, amantadine, benztropine, bromocriptine, disopyramide, levodopa, selegiline, pergolide, procyclidine, sedating antihistamines, phenothiazines, tricyclic antidepressants, orphenadrine) Phenytoin Alteration in phenytoin metabolism. Plasma levels may be increased or decreased. Monitor.Phenothiazines may also reduce seizure threshold Parkinson’s drugs (eg, cabergoline, levodopa, lisuride) SSRI and tricyclic antidepressants Effects of Parkinson’s disease drugs may be antagonised since prochlorperazine has some dopamine antagonist activity Metabolism may be inhibited increasing risk of adverse effects (eg, extrapyramidal reactions, sedation, possibly QT prolongation) Sildenafil Alpha blockers (eg, doxazosin) May reduce both standing and supine blood pressure. Dizziness reported CYP 3A4 (eg, cimetidine, erythromycin, Drugs that inhibit these enzymes may increase sildenafil concentrations saquinavir) Nitrates (eg, glyceryl trinitrate, Contraindicated. Potentiates hypotensive effects isosorbide dinitrate) Sumatriptan, zolmitriptan Ergotamine or related compounds Additive vasoconstriction may result in fatal reactions (eg, MI). Avoid combination MAOIs – irreversible (eg, phenelzine, tranylcypromine) MAOIs – reversible (eg, moclobemide) Medications that increase serotonin levels (eg, SSRIs, venlafaxine, tricyclic antidepressants sibutramine, St John’s wort) Medications that lower the seizure threshold (eg, neuroleptics, antidepressants) MAOIs can inhibit metabolism of sumatriptan increasing risk of serotonin syndrome. Avoid for 2 weeks after stopping MAOI Moclobemide inhibits metabolism of sumatriptan. Increased risk of serotonin syndrome. Avoid Increased risk of serotonin syndrome. Avoid combination or careful monitoring required Sumatriptan can also lower seizure threshold so additive effects possible Page 189

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