May potentiate adverse reactions w/ other long-acting muscarinic antagonists & β
2-adrenergic agonists. May potentiate possible hypokalaemic effect w/ methylxanthine derivatives, steroids or non-K-sparing diuretics. Vilanterol: May weaken or antagonize effect w/ either non-selective or selective β-adrenergic blockers. May inhibit metabolism & increased systemic exposure w/ strong CYP3A4 inhibitors (eg, ketoconazole, clarithromycin, itraconazole, ritonavir, telithromycin).