Medically reviewed on July 2, 2018
What is methenamine?
Methenamine is an antibiotic that fights bacteria.
Methenamine is used to treat or prevent recurring bladder infection in adults and children who are at least 12 years old.
Methenamine is usually given after other medicines have been tried without success.
Methenamine may also be used for purposes not listed in this medication guide.
You should not use methenamine if you have kidney disease, severe liver disease, if you also take a sulfa drug, or if you are dehydrated.
Before taking this medicine
You should not use methenamine if you are allergic to it, or if you have:
severe liver disease;
if you are dehydrated.
To make sure methenamine is safe for you, tell your doctor if you have:
FDA pregnancy category C. It is not known whether methenamine will harm an unborn baby. Tell your doctor if you are pregnant or plan to become pregnant while using methenamine.
Methenamine can pass into breast milk and may harm a nursing baby. Tell your doctor if you are breast-feeding a baby.
How should I take methenamine?
Follow all directions on your prescription label. Do not take this medicine in larger or smaller amounts or for longer than recommended.
Taking more of this medicine than prescribed will not make it more effective. Taking too much methenamine may cause unwanted side effects, such as bladder irritation, pain, increased urination, and blood in the urine.
Drink plenty of liquids while you are taking methenamine. Your doctor may recommend drinking certain types of liquid (such as cranberry juice) or taking other medicines to make your urine more acidic. Your doctor may also recommend increasing protein in your diet, but avoiding milk or other dairy products. Follow your doctor's instructions very closely.
You may need frequent urine tests to determine that an infection has cleared. You may also need blood tests to check your liver function.
Use this medicine for the full prescribed length of time. Your symptoms may improve before the infection is completely cleared. Skipping doses may also increase your risk of further infection that is resistant to antibiotics. Methenamine will not treat a viral infection such as the flu or a common cold.
Do not use methenamine to treat any condition that has not been checked by your doctor.
Do not share this medicine with another person, even if they have the same symptoms you have.
Store at room temperature away from moisture and heat.
What happens if I miss a dose?
Take the missed dose as soon as you remember. Skip the missed dose if it is almost time for your next scheduled dose. Do not take extra medicine to make up the missed dose.
What happens if I overdose?
Seek emergency medical attention or call the Poison Help line at 1-800-222-1222.
What should I avoid while taking methenamine?
Methenamine may be only part of a complete program of treatment that also includes a special diet. Follow the diet plan created for you by your doctor or nutrition counselor. Get familiar with the list of foods you must eat or avoid to help control your condition.
You may need to avoid citrus fruits such as oranges, grapefruits, and lemons (including juices).
Ask your doctor before using an antacid, and use only the type your doctor recommends. Some antacids can make it harder for your body to absorb methenamine.
Methenamine side effects
Get emergency medical help if you have any of these signs of an allergic reaction: hives; difficult breathing; swelling of your face, lips, tongue, or throat.
Call your doctor at once if you have any new or worsening symptoms.
Common side effects may include:
This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.
See also: Side effects (in more detail)
What other drugs will affect methenamine?
Other drugs may interact with methenamine, including prescription and over-the-counter medicines, vitamins, and herbal products. Tell each of your health care providers about all medicines you use now and any medicine you start or stop using.
Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use this medication only for the indication prescribed.
Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.
Copyright 1996-2018 Cerner Multum, Inc. Version: 2.01.
More about methenamine
- Methenamine Side Effects
- During Pregnancy or Breastfeeding
- Dosage Information
- Drug Images
- Drug Interactions
- Support Group
- Pricing & Coupons
- En Español
- 32 Reviews
- Drug class: urinary anti-infectives