Drugs List

Therapeutic Indications

Uses

Prophylaxis of recurrent urinary tract infections
Urinary tract infection

Prophylaxis and treatment of urinary tract infections including:

As maintenance therapy after successful initial treatment of acute infections with antibiotics

As long-term therapy in the prevention of recurrent cystitis

For suppression of urinary infection in patients with indwelling catheters and to reduce the incidence of catheter blockage

For prophylaxis against the introduction of infection into the urinary tract during instrumental procedures

Asymptomatic bacteriuria

Contraindications

Children under 6 years
Gout
Hepatic impairment
Metabolic acidosis
Renal impairment - glomerular filtration rate below 10ml/minute
Renal parenchymal infection
Severe dehydration

Precautions and Warnings

Pregnancy

May affect results of some laboratory tests
Advise patient not to self-medicate with urinary alkalinising agents

Pregnancy and Lactation

Pregnancy

Use with caution during pregnancy.

There is conflicting evidence on the use of methenamine during pregnancy. The manufacturer states that although there is inadequate evidence of safety of methenamine use during human pregnancy, it has been in wide use for many years without apparent ill consequence. Animal studies have shown no hazard. Schaefer (2007) states that methenamine is contraindicated during pregnancy although adverse effects on pregnancy outcomes have not been reported. Schaefer adds that inadvertent use is not an indication for termination of pregnancy or for invasive prenatal diagnostic procedures. Briggs (2011), however, concludes that methenamine is compatible with pregnancy, studies showed no increases in congenital defects or other problems.

The use of all medication in pregnancy should be avoided whenever possible; particularly in the first trimester. Non-drug treatments should also be considered. When essential, a medication with the best safety record over time should be chosen, employing the lowest effective dose for the shortest possible time. Polypharmacy should be avoided. Teratogens taken in the pre-embryonic period, often quoted as lasting until 14-17 days post-conception, are believed to have an all-or-nothing effect. Where drugs have a short half-life, and when the date of conception is certain, this may allow women to be reassured where drug exposure has occurred within this time frame. Further advice may be available from the UK National Teratology Information Service (NTIS) and through ToxBase, available via password on the internet ( www.toxbase.org ) or if this is unavailable at the backup site ( www.toxbasebackup.org ).

Lactation

Methenamine is considered safe for use in breastfeeding. Methenamine is excreted in breast milk with peak levels occurring at 1 hour but the quantities will be insignificant to the infant. Mothers can therefore breastfeed their infants.

Neonates, infants born prematurely, those with low birth weight, those with an unstable gastrointestinal function or who have serious illnesses may require special consideration. For any infant, if a drug is prescribed to the nursing mother, it should be at the lowest practical dose and for the shortest time. When drug administration is unavoidable and breastfeeding is to continue, minimisation of exposure of the infant to the drug may sometimes be achieved by timing the maternal doses to just after a feeding episode. Infants exposed to drugs via breast milk should be monitored for unusual signs or symptoms. Interactions between the drug received by the infant from the mother's milk and medication prescribed for the infant should also be considered, for example, when the drug given to the infant may prevent metabolism of the drug received via breast milk.
Specialist advice is available from the UK Drugs in Lactation Advisory Service at https://www.midlandsmedicines.nhs.uk/content.asp?section=6&subsection=17&pageIdx=1

Side Effects

Bladder irritation
Gastro-intestinal disturbances
Pruritus
Rash

Presentation

Tablets containing methenamine

Drugs List

Therapeutic Indications

Uses

Prophylaxis of recurrent urinary tract infections
Urinary tract infection

Prophylaxis and treatment of urinary tract infections including:

As maintenance therapy after successful initial treatment of acute infections with antibiotics

As long-term therapy in the prevention of recurrent cystitis

For suppression of urinary infection in patients with indwelling catheters and to reduce the incidence of catheter blockage

For prophylaxis against the introduction of infection into the urinary tract during instrumental procedures

Asymptomatic bacteriuria

Dosage

Adults

Elderly

Children

Contraindications

Children under 6 years
Gout
Hepatic impairment
Metabolic acidosis
Renal impairment - glomerular filtration rate below 10ml/minute
Renal parenchymal infection
Severe dehydration

Precautions and Warnings

Pregnancy

May affect results of some laboratory tests
Advise patient not to self-medicate with urinary alkalinising agents

Pregnancy and Lactation

Pregnancy

Use with caution during pregnancy.

There is conflicting evidence on the use of methenamine during pregnancy. The manufacturer states that although there is inadequate evidence of safety of methenamine use during human pregnancy, it has been in wide use for many years without apparent ill consequence. Animal studies have shown no hazard. Schaefer (2007) states that methenamine is contraindicated during pregnancy although adverse effects on pregnancy outcomes have not been reported. Schaefer adds that inadvertent use is not an indication for termination of pregnancy or for invasive prenatal diagnostic procedures. Briggs (2011), however, concludes that methenamine is compatible with pregnancy, studies showed no increases in congenital defects or other problems.

The use of all medication in pregnancy should be avoided whenever possible; particularly in the first trimester. Non-drug treatments should also be considered. When essential, a medication with the best safety record over time should be chosen, employing the lowest effective dose for the shortest possible time. Polypharmacy should be avoided. Teratogens taken in the pre-embryonic period, often quoted as lasting until 14-17 days post-conception, are believed to have an all-or-nothing effect. Where drugs have a short half-life, and when the date of conception is certain, this may allow women to be reassured where drug exposure has occurred within this time frame. Further advice may be available from the UK National Teratology Information Service (NTIS) and through ToxBase, available via password on the internet ( www.toxbase.org ) or if this is unavailable at the backup site ( www.toxbasebackup.org ).

Lactation

Methenamine is considered safe for use in breastfeeding. Methenamine is excreted in breast milk with peak levels occurring at 1 hour but the quantities will be insignificant to the infant. Mothers can therefore breastfeed their infants.

Neonates, infants born prematurely, those with low birth weight, those with an unstable gastrointestinal function or who have serious illnesses may require special consideration. For any infant, if a drug is prescribed to the nursing mother, it should be at the lowest practical dose and for the shortest time. When drug administration is unavoidable and breastfeeding is to continue, minimisation of exposure of the infant to the drug may sometimes be achieved by timing the maternal doses to just after a feeding episode. Infants exposed to drugs via breast milk should be monitored for unusual signs or symptoms. Interactions between the drug received by the infant from the mother's milk and medication prescribed for the infant should also be considered, for example, when the drug given to the infant may prevent metabolism of the drug received via breast milk.
Specialist advice is available from the UK Drugs in Lactation Advisory Service at https://www.midlandsmedicines.nhs.uk/content.asp?section=6&subsection=17&pageIdx=1

Counselling

Tablets may be halved or crushed and taken with a drink of milk or fruit juice.

Advise patient not to self-medicate with urinary alkalinising agents.

Side Effects

Bladder irritation
Gastro-intestinal disturbances
Pruritus
Rash

Effects on Laboratory Tests

Interference with laboratory test estimations for steroids, catecholamines and 5 hydroxyindole acetic acid in the urine has been reported.

Overdosage

It is strongly recommended that the UK National Poisons Information Service be consulted on cases of suspected or actual overdose where there is doubt over the degree of risk or about appropriate management.

The following number will direct the caller to the relevant local centre (0844) 892 0111

Information may be obtained if you have access to ToxBase the primary clinical toxicology database of the National Poisons Information Service. This is available via password on the internet ( www.toxbase.org ) or if this is unavailable at the backup site ( www.toxbasebackup.org ).

Further Information

Last Full Review Date: June 2013

Reference Sources

British National Formulary, 65th Edition (March - September 2013) Pharmaceutical Press, London.

Drugs During Pregnancy and Lactation: Treatment Options and Risk Assessment, 2nd edition (2007) ed. Schaefer, C., Peters, P. and Miller, R. Elsevier, London.

Drugs in Pregnancy and Lactation: A Reference Guide to Fetal and Neonatal Risk, 9th edition (2011) ed. Briggs, G., Freeman, R. and Yaffe, S. Lippincott Williams & Wilkins, Philadelphia.

Summary of Product Characteristics: Hiprex tablets. Meda Pharmaceuticals Ltd. Revise March 2018.

US National Library of Medicine. Toxicology Data Network. Drugs and Lactation Database (LactMed).
Available at: https://toxnet.nlm.nih.gov/cgi-bin/sis/htmlgen?LACT
Methenamine. Last revised: December 7, 2010.
Last accessed: June 12, 2013.