Metenamine: What is it? Administration, Precautions, Side Effects and Interactions

This medication is commonly prescribed to adults and children who are over 12 years of age to treat or prevent bladder infections.

Methenamine is an antibiotic that fights microorganisms.

It is usually given after other medications have been tried without success.

Before taking this medicine

You should not use methenamine if you are allergic to it or if you have:

  • Kidney disease
  • Severe liver disease.
  • If you also take a sulfa medication (Azulfidine, Bactrim, Septra, SMZ-TMP, Sulfazine, and others).
  • If you are dehydrated.

To make sure Metenamine is safe for you, tell your doctor if you have:

  • Liver disease
  • Asthma.
  • Some allergies to aspirin.
  • If you are allergic to yellow food coloring.

It is not known if Metenamine can cause harm to the fetus. Notify your doctor if you are pregnant or plan to become pregnant while taking this medicine.

Methenamine may be passed on to breast milk and harm a nursing infant. Tell your doctor if you are breastfeeding a baby.



Follow all instructions prescribed on the label. Do not take this medication in greater or lesser amounts than those recommended by the doctor. Do not eat for longer than indicated.

Taking methenamine in excess may cause unwanted side effects, such as:

  • Pain.
  • Irritation of the bladder.
  • Increase in urine.
  • Presence of blood in the urine.

Your doctor may recommend drinking certain liquids or taking other medications to make the urine more acidic. Your doctor can also ban your milk or other dairy products and recommend a diet rich in protein.

It is essential to follow your doctor’s instructions at the bottom of the letter. You may need frequent urine tests to determine if an infection has been removed. You may also need blood tests to monitor your liver function.


  • Use this medication only for the full prescribed time.
  • Please consider that the symptoms may improve before the infection has completely disappeared, so it is essential to continue with the treatment.
  • Omitting the indicated doses may increase the risk of further infection that may be resistant to antibiotics.
  • Methenamine is not recommended to treat a viral infection such as the flu or a common cold.
  • Taking an extra dose of the prescription will not make it more effective.
  • Do not take methenamine to treat any condition your doctor has not previously checked.
  • Store at room temperature away from moisture and heat.

What should you do if you miss a dose?

  • Take the missed dose as soon as you remember it. Skip the missed dose if it is almost time for your next scheduled dose.
  • Do not take extra medicine to make up for the forgotten dose.
  • If you swallow an overdose of Metenamine, seek emergency medical attention.

What should I avoid being treated with methenamine?

This medication may only be part of a complete treatment plan that includes a strict diet. Follow the diet plan created by your doctor or nutrition counselor for you.

Know the list of foods you should eat or avoid to help control your condition. You may need to avoid citrus fruits (including juices).

Consult your doctor before taking an antacid, and use only the type recommended by your doctor. Some antacids can make it harder for your body to absorb methenamine.

Side effects

Seek immediate medical attention if you experience any of these symptoms:

  • Swelling of the face, tongue, lips, or throat.
  • Difficulty breathing.
  • Skin rash.

Call your doctor right away if you have new or worsening symptoms.

Common side effects may include:

  • Painful or difficult urination
  • Stomach ache.
  • Sickness.
  • Eruption.


Intake of other medications may interact with methenamine, including over-the-counter medicines, such as vitamins and natural herbal products.

Tell each of your health care providers about all medications you currently use and any medications you start or stop using.