Ketoconazole and hyoscyamine may cause increased gastrointestinal pH. Concurrent administration with hyoscyamine may result in marked reduction in the absorption of ketoconazole. Patients should be advised to take this combination at least 2 hours after ketoconazole.
Source: FDA drug label - methenamine, sodium phosphate, monobasic, monohydrate, phenyl salicylate, methylene blue, and hyoscyamine sulfate