Cinnarizine may enhance the sedative effect of CNS depressants including alcohol, barbiturates, hypnotics, narcotic analgesics, tricyclic antidepressants, sedative and tranquilisers.
The side effects of anticholinergic substances eg, atropine and tricyclic antidepressants may be enhanced by the concomitant administration of antihistamines.
Monoamine oxidase inhibitors may enhance the antimuscarinic effects of antihistamines.
Domperidone: Concomitant administration of anticholinergic drugs may inhibit the anti-dyspeptic effects of domperidone. Antimuscarinic agents and opioid analgesics may antagonise the effect of domperidone. Domperidone suppresses the peripheral effects (digestive disorders, nausea and vomiting) of dopaminergic agonists. Since domperidone has gastrokinetic effects, it could influence the absorption of concomitant orally administered medicines, particularly those with sustained-release or enteric-coated formulations. As domperidone interferes with serum prolactin levels, it may interfere with other hypoprolactinaemic agents and with some diagnostic tests.
Antacids and antisecretory agents lower the oral bioavailability of domperidone. They should be taken after meals and not before meals ie, they should not be taken simultaneously with domperidone.
Reduced gastric acidity impairs the absorption of domperidone.
Oral bioavailability is decreased by prior administration of cimetidine or sodium bicarbonate.
The main metabolic pathway of domperidone is through CYP3A4. In vitro data suggests that concomitant use of drugs that significantly inhibit this enzyme may result in increased plasma levels of domperidone. Examples of CYP3A4 inhibitors include: Azole antifungals, macrolide antibiotics, HIV-protease inhibitors, nefazodone.