Metabolized by CYP isoenzymes, principally CYP2D6 and CYP3A4. 
Does not inhibit CYP1A1, 1A2, 2B6, C29, 2C19, 2D6, 2E1, or 3A4.
Specific Drugs
Drug
Interaction
Comments
β2-Adrenergic agonists
No adverse drug interactions reported 
Antihistamines
No adverse drug interactions reported
Anti-IgE monoclonal antibody therapy
No adverse drug interactions reported
Antimuscarinic agents
Potential increase in anticholinergic effects 
Concomitant use not recommended by manufacturer 
Corticosteroids, oral and inhaled
No adverse drug interactions reported
Histamine H2-receptor antagonists
Increased AUC and decreased renal clearance of IV tiotropium (not currently available in the US) with concomitant cimetidine but not ranitidine
Pharmacokinetic interactions not considered clinically important
Leukotriene modifiers
No adverse drug interactions reported
Mast-cell stabilizers
No adverse drug interactions reported.
Methylxanthines
No adverse drug interactions reported 
Mucolytic agents
No adverse drug interactions reported
 Tiotropium drug interactions (more detail)
