In the U.S.—

In Canada—

• Antibacterial, systemic

Methenamine (meth-EN-a-meen) belongs to the family of medicines called anti-infectives. It is used to help prevent and treat infections of the urinary tract. Methenamine is available only with your doctor's prescription, in the following dosage forms:

Oral
• Enteric-coated tablets (U.S.)
• Granules for oral solution (U.S.)
• Oral suspension (U.S.)
• Tablets (U.S. and Canada)

In deciding to use a medicine, the risks of taking the medicine must be weighed against the good it will do. This is a decision you and your doctor will make. For methenamine, the following should be considered:

Allergies—Tell your doctor if you have ever had any unusual or allergic reaction to methenamine. Also tell your health care professional if you are allergic to any other substances, such as foods, preservatives, or dyes.

Pregnancy—Methenamine has not been studied in either humans or animals. However, individual case reports on the use of methenamine during pregnancy have not shown that this medicine causes birth defects or other problems in humans.

Breast-feeding—Methenamine passes into the breast milk. However, methenamine has not been reported to cause problems in nursing babies.

Children—Although there is no special information comparing use of methenamine in children with use in other age groups, this medicine is not expected to cause different side effects or problems in children than it does in adults.

Older adults—Many medicines have not been studied specifically in older people. Therefore, it may not be known whether they work exactly the same way they do in younger adults or if they cause different side effects or problems in older people. There is no specific information comparing use of methenamine in the elderly with use in other age groups.

Other medicines—Although certain medicines should not be used together at all, in other cases two different medicines may be used together even if an interaction might occur. In these cases, your doctor may want to change the dose, or other precautions may be necessary. When you are taking methenamine, it is especially important that your health care professional knows if you are taking any of the following:

• Sulfamethizole (use of methenamine with this medicine may damage your kidneys)
• Thiazide diuretics (water pills) or
• Urinary alkalizers (medicine that makes the urine less acid, such as acetazolamide [e.g., Diamox], calcium- and/or magnesium-containing antacids, dichlorphenamide [e.g., Daranide], methazolamide [e.g., Neptazane], potassium or sodium citrate and/or citric acid, sodium bicarbonate [baking soda])—Use of methenamine with any of these medicines may decrease the effectiveness of methenamine

• Dehydration (severe) or
• Kidney disease (severe)—Patients with severe kidney disease who take methenamine may have an increase in side effects that affect the kidneys

• Liver disease (severe)—Patients with severe liver disease who take methenamine may have an increase in symptoms of their liver disease

Before you start taking this medicine, check your urine with phenaphthazine paper or another test to see if it is acid. Your urine must be acidic (pH 5.5 or below) for this medicine to work properly. If you have any questions about this, check with your health care professional.

The following changes in your diet may help make your urine more acid; however, check with your doctor first if you are on a special diet (for example, for diabetes). Avoid most fruits (especially citrus fruits and juices), milk and other dairy products, and other foods that make the urine less acid. Also, avoid antacids unless otherwise directed by your doctor. Eating more protein and foods such as cranberries (especially cranberry juice with vitamin C added), plums, or prunes may also help. If your urine is still not acid enough, check with your doctor.

If this medicine causes nausea or upset stomach, it may be taken after meals and at bedtime.

For patients taking the dry granule form of this medicine :

• Dissolve the contents of each packet in 2 to 4 ounces of cold water immediately before taking. Stir well. Be sure to drink all the liquid to get the full dose of medicine.

For patients taking the oral liquid form of this medicine :

• Use a specially marked measuring spoon or other device to measure each dose accurately. The average household teaspoon may not hold the right amount of liquid.

For patients taking the enteric-coated tablet form of this medicine:

• Swallow tablets whole. Do not break, crush, or take if chipped.

To help clear up your infection completely, keep taking this medicine for the full time of treatment, even if you begin to feel better after a few days. Do not miss any doses.

Dosing—

The dose of methenamine will be different for different patients. Follow your doctor's orders or the directions on the label. The following information includes only the average doses of methenamine. If your dose is different, do not change it unless your doctor tells you to do so.

The number of tablets or teaspoonfuls of solution or suspension that you take depends on the strength of the medicine.

• For the treatment of urinary tract infections:
  • For oral dosage form (methenamine hippurate tablets):
    • Adults and children 12 years of age and over—1 gram two times a day. Take in the morning and the evening.
    • Children up to 6 years of age—Use and dose must be determined by your doctor.
    • Children 6 to 12 years of age—500 milligrams (mg) to 1 gram two times a day. Take in the morning and the evening.
  • For oral dosage form (methenamine mandelate enteric-coated tablets, regular tablets, solution, and suspension):
    • Adults and children 12 years of age and over—1 gram four times a day. Take after meals and at bedtime.
    • Children up to 6 years of age—Dose is based on body weight. The usual dose is 18.3 mg per kilogram (kg) (8.3 mg per pound) of body weight four times a day. Take after meals and at bedtime.
    • Children 6 to 12 years of age—500 mg four times a day. Take after meals and at bedtime.

Missed dose—

If you miss a dose of this medicine, take it as soon as possible. However, if it is almost time for your next dose, skip the missed dose and go back to your regular dosing schedule. Do not double doses.

Storage—

To store this medicine:

• Keep out of the reach of children.
• Store away from heat and direct light.
• Do not store the dry granule or tablet form of this medicine in the bathroom, near the kitchen sink, or in other damp places. Heat or moisture may cause the medicine to break down.
• Keep the oral liquid form of this medicine from freezing.
• Do not keep outdated medicine or medicine no longer needed. Be sure that any discarded medicine is out of the reach of children.

If your symptoms do not improve within a few days, or if they become worse, check with your doctor.

Along with its needed effects, a medicine may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.

Check with your doctor immediately if any of the following side effects occur:

• Less common
  • Skin rash 
  
  

• Rare
  • Blood in urine;  lower back pain;  pain or burning while urinating 
  
  

Other side effects may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. However, check with your doctor if any of the following side effects continue or are bothersome:

• Less common
  • Nausea and vomiting 
  
  

Other side effects not listed above may also occur in some patients. If you notice any other effects, check with your doctor.