SIDE EFFECTS: Nausea, vomiting, diarrhea, abdominal cramps, and loss of appetite may occur. If any of these effects persist or worsen, tell your doctor or pharmacist promptly. Painful or difficult urination may occur with this medication, although less often. Report these side effects to your doctor, who may need to lower your dose of this medication or change your treatment to make your urine less acidic. Remember that your doctor has prescribed this medication because he or she has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects. Taking higher than recommended doses of methenamine may cause bladder irritation, painful/frequent urination, and bloody/pink urine. Tell your doctor right away if you have any of these symptoms. Tell your doctor right away if any of these unlikely but serious side effects occur: mouth sores, unusual headache, ringing in the ears, muscle cramps, swelling of the arms/legs/feet. Tell your doctor right away if any of these rare but serious side effects occur: trouble breathing (such as rapid breathing, shortness of breath), confusion, persistent vomiting, bone/joint pain, dizziness, seizures, unusual tiredness/weakness, numbness/tingling/pain/weakness of hands/feet/legs, numbness/tingling around lips, fast/irregular heartbeat, unusual weight gain, signs of kidney problems (such as change in the amount of urine), unusual thirst. A very serious allergic reaction to this drug is rare. However, seek immediate medical attention if you notice any symptoms of a serious allergic reaction, including: rash, itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing. This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist. In the US - Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088 or at www.fda.gov/medwatch. In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.

PRECAUTIONS: Before taking this medication, tell your doctor or pharmacist if you are allergic to methenamine or sodium acid phosphate; or to formaldehyde; or if you have any other allergies. This product may contain inactive ingredients, which can cause allergic reactions or other problems. Talk to your pharmacist for more details. Before using this medication, tell your doctor or pharmacist your medical history, especially of: kidney problems, liver problems, dehydration, high blood levels of certain minerals (sodium or phosphate), heart failure, swelling of ankles/feet, fluid in the lungs (pulmonary edema), high blood pressure, high blood pressure during pregnancy, parathyroid problems, disease of the pancreas (pancreatitis), metabolic imbalance (metabolic acidosis). This product contains a large amount of sodium. It should be used with caution if you are on a low-sodium diet. Before having surgery, tell your doctor or dentist about all the products you use (including prescription drugs, nonprescription drugs, and herbal products). During pregnancy, this medication should be used only when clearly needed. Discuss the risks and benefits with your doctor. This drug passes into breast milk and the effect on a nursing infant is unknown. Consult your doctor before breast-feeding.

DRUG INTERACTIONS: See also How to Use section. The effects of some drugs can change if you take other drugs or herbal products at the same time. This can increase your risk for serious side effects or may cause your medications not to work correctly. These drug interactions are possible, but do not always occur. Your doctor or pharmacist can often prevent or manage interactions by changing how you use your medications or by close monitoring. To help your doctor and pharmacist give you the best care, be sure to tell your doctor and pharmacist about all the products you use (including prescription drugs, nonprescription drugs, and herbal products) before starting treatment with this product. While using this product, do not start, stop, or change the dosage of any other medicines you are using without your doctor's approval. Some products that may interact with this drug include: sulfonamide drugs (including sulfa antibiotics such as sulfamethizole), corticosteroids (such as corticotropin, fludrocortisone), products that decrease the amount of acid in the urine (urinary alkalinizers such as antacids, sodium bicarbonate, potassium or sodium citrate, carbonic anhydrase inhibitors such as acetazolamide). Antacids containing aluminum, calcium, or magnesium can also bind to phosphate, which can cause this drug to be less effective. This medication when used with salicylates (such as high doses of aspirin) may lead to increased blood salicylate levels and possible side effects. If you are taking high doses of salicylates, tell your doctor so your treatment can be adjusted. If your doctor has directed you to take low-dose aspirin for heart attack or stroke prevention (usually at dosages of 81-325 milligrams a day), you should continue taking it unless your doctor instructs you otherwise. Ask your doctor or pharmacist for more details. This medication may interfere with certain laboratory tests, possibly causing false test results. Make sure laboratory personnel and all your doctors know you use this drug. This document does not contain all possible drug interactions. Keep a list of all the products you use. Share this list with your doctor and pharmacist to lessen your risk for serious medication problems.

OVERDOSE: If someone has overdosed and has serious symptoms such as passing out or trouble breathing, call 911. Otherwise, call a poison control center right away. US residents can call their local poison control center at 1-800-222-1222. Canada residents can call a provincial poison control center.