Description and Brand Names
Drug information provided by: Merative, Micromedex®
US Brand Name
Canadian Brand Name
Methenamine 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.
This product is available in the following dosage forms:
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 this medicine, the following should be considered:
Tell your doctor if you have ever had any unusual or allergic reaction to this medicine or any other medicines. Also tell your health care professional if you have any other types of allergies, such as to foods, dyes, preservatives, or animals. For non-prescription products, read the label or package ingredients carefully.
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.
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.
There are no adequate studies in women for determining infant risk when using this medication during breastfeeding. Weigh the potential benefits against the potential risks before taking this medication while breastfeeding.
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 this medicine, it is especially important that your healthcare professional know if you are taking any of the medicines listed below. The following interactions have been selected on the basis of their potential significance and are not necessarily all-inclusive.
Using this medicine with any of the following medicines is not recommended. Your doctor may decide not to treat you with this medication or change some of the other medicines you take.
Certain medicines should not be used at or around the time of eating food or eating certain types of food since interactions may occur. Using alcohol or tobacco with certain medicines may also cause interactions to occur. The following interactions have been selected on the basis of their potential significance and are not necessarily all-inclusive.
Other Medical Problems
The presence of other medical problems may affect the use of this medicine. Make sure you tell your doctor if you have any other medical problems, especially:
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.
The dose of this medicine 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 this medicine. If your dose is different, do not change it unless your doctor tells you to do so.
The amount of medicine that you take depends on the strength of the medicine. Also, the number of doses you take each day, the time allowed between doses, and the length of time you take the medicine depend on the medical problem for which you are using 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.
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.
Store the medicine in a closed container at room temperature, away from heat, moisture, and direct light. Keep from freezing.
Keep out of the reach of children.
Do not keep outdated medicine or medicine no longer needed.
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:
Blood in urine
lower back pain
pain or burning while urinating
Some 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. Also, your health care professional may be able to tell you about ways to prevent or reduce some of these side effects. Check with your health care professional if any of the following side effects continue or are bothersome or if you have any questions about them:
Nausea and vomiting
Other side effects not listed may also occur in some patients. If you notice any other effects, check with your healthcare professional.
Call your doctor for medical advice about side effects. You may report side effects to the FDA at 1-800-FDA-1088.
Portions of this document last updated: Feb. 01, 2023
Original article: https://www.mayoclinic.org/drugs-supplements/methenamine-oral-route/side-effects/drg-20064676
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