2017 HCHB_digital

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
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