Gastrointestinal disturbances (nausea, stomach upset), generalized skin rash, dysuria, painful or difficult urination may occur occasionally with the use of methenamine preparations. Microscopic and rarely, gross hematuria have also been reported.
Gastrointestinal upset (diarrhea, nausea, stomach pain, and vomiting) may occur with the use of sodium phosphates. Also, bone or joint pain (possible phosphate induced osteomalacia) could occur. The following adverse effects may be observed (primarily from sodium): headaches; dizziness; mental confusion; seizures; weakness or heaviness of legs; unusual tiredness or weakness; muscle cramps; numbness, tingling, pain, or weakness of hands or feet; numbness or tingling around lips; fast or irregular heartbeat; shortness of breath or troubled breathing; swelling of feet or lower legs; unusual weight gain; low urine output, unusual thirst.