Dosing & Uses
Dosage Forms & Strengths
methenamine/sodium acid phosphate
tablet
- 500 mg/500 mg
Urinary Tract Infection Prophylaxis
Initial: 2 tablets PO q6hr with liberal fluid intake
Maintenance: 1-2 tablets PO q12hr
Use only after eradication of UTI by other appropriate antibiotic
Methenamine: Indicated for prophylaxis/suppression for chronic recurring UTIs when long-term therapy is required
Sodium acid phosphate: Urinary acidifier
Safety/efficacy not established
Interactions
Interaction Checker
No Results
Contraindicated
Serious - Use Alternative
Significant - Monitor Closely
Minor
Contraindicated (1)
- sodium phosphate rectal
sodium phosphate rectal, sodium acid phosphate. Either increases effects of the other by pharmacodynamic synergism. Contraindicated. Either increases toxicity of the other. Coadministration of various sodium phosphate preparations are known to cause electrolyte disturbances and hypovolemia.
Serious - Use Alternative (6)
- acetazolamide
acetazolamide, methenamine. Other (see comment). Contraindicated. Comment: This combination may form an insoluble precipitate in the urine, decreasing the effects of both agents.
- baloxavir marboxil
sodium acid phosphate will decrease the level or effect of baloxavir marboxil by cation binding in GI tract. Avoid or Use Alternate Drug. Baloxavir may bind to polyvalent cations resulting in decreased absorption. Studies in monkeys showed concurrent use with calcium, aluminum, or iron caused significantly decreased plasma levels. Human studies not conducted.
- cholera vaccine
methenamine, cholera vaccine. pharmacodynamic antagonism. Avoid or Use Alternate Drug. Avoid coadministration of cholera vaccine with systemic antibiotics since these agents may be active against the vaccine strain. Do not administer cholera vaccine to patients who have received oral or parenteral antibiotics within 14 days prior to vaccination.
- microbiota oral
methenamine decreases effects of microbiota oral by pharmacodynamic antagonism. Avoid or Use Alternate Drug. Microbiota oral contains bacterial spores. Antibacterial agents may decrease efficacy if coadministered. Complete antibiotic regimens 2-4 days before initiating microbiota oral. .
- potassium phosphates, IV
sodium acid phosphate and potassium phosphates, IV both increase serum potassium. Avoid or Use Alternate Drug.
- sulfadiazine
methenamine, sulfadiazine. Other (see comment). Contraindicated. Comment: This combination may form an insoluble precipitate in the urine, decreasing the effects of both agents.
Monitor Closely (9)
- deflazacort
sodium acid phosphate and deflazacort both decrease serum potassium. Use Caution/Monitor.
- dextroamphetamine
sodium acid phosphate decreases levels of dextroamphetamine by increasing renal clearance. Use Caution/Monitor.
- lisdexamfetamine
sodium acid phosphate decreases levels of lisdexamfetamine by increasing renal clearance. Use Caution/Monitor.
- methamphetamine
sodium acid phosphate decreases levels of methamphetamine by increasing renal clearance. Use Caution/Monitor.
- methylenedioxymethamphetamine
sodium acid phosphate decreases levels of methylenedioxymethamphetamine by increasing renal clearance. Use Caution/Monitor.
- omadacycline
sodium acid phosphate will decrease the level or effect of omadacycline by inhibition of GI absorption. Applies only to oral form of both agents. Modify Therapy/Monitor Closely. Multivalent cation-containing products may impair absorption of tetracyclines, which may decrease its efficacy. Separate dosing of tetracyclines from these products.
- pseudoephedrine
methenamine decreases effects of pseudoephedrine by unknown mechanism. Use Caution/Monitor. Urinary excretion of indirect acting alpha/beta agonists (eg, pseudoephedrine) may increase when administered concomitantly with urinary acidifying agents, resulting in lower serum concentrations.
- sarecycline
sodium acid phosphate will decrease the level or effect of sarecycline by inhibition of GI absorption. Applies only to oral form of both agents. Modify Therapy/Monitor Closely. Multivalent cation-containing products may impair absorption of tetracyclines, which may decrease its efficacy. Separate dosing of tetracyclines from these products.
- sodium picosulfate/magnesium oxide/anhydrous citric acid
methenamine decreases effects of sodium picosulfate/magnesium oxide/anhydrous citric acid by altering metabolism. Use Caution/Monitor. Coadministration with antibiotics decreases efficacy by altering colonic bacterial flora needed to convert sodium picosulfate to active drug.
Minor (0)
Adverse Effects
1-10%
Dysuria
Gastric upset
Nausea
Rash
Frequency Not Defined
Diarrhea
Nausea
Stomach pain
Vomiting
Warnings
Black Box Warnings
Sodium acid phosphate
- Rare reports of acute phosphate nephropathy with oral sodium phosphate products used for colon cleansing before colonoscopy
- Some cases have resulted in permanent renal function impairment requiring long-term hemodialysis
- Risk factors for acute phosphate nephropathy include age >55 yr, hypovolemia, baseline kidney disease, bowel obstruction, active colitis, and those using medicines that affect renal perfusion or function (eg, diuretics, ACE inhibitors, ARBs, NSAIDs)
- Carefully follow dosing regimen as recommended (pm/am split dose) with adequate hydration
Contraindications
Methenamine
- Hypersensitivity to methenamine or tartrazine (FD&C Yellow No. 5)
- Renal or severe hepatic insufficiency
- Concurrent sulfonamides or acetazolamide may form insoluble precipitate in urine
Sodium acid phosphate
- Hypersensitivity
- Addison's disease
- Hyperphosphatemia
- Acidification of urine in urinary stone disease
- Urolithiasis or struvite stone formation
- Severe renal impairment (ie, CrCl <30 mL/min)
Cautions
Methenamine
- Large doses may cause bladder irritation, urinary frequency, albuminuria, and hematuria
- Maintain acidic pH of urine, especially when treating urea-splitting organisms (eg, Proteus, Pseudomonas)
- Monitor LFTs, especially with history of liver impairment
- Safe use not established during pregnancy, especially 1st and 2nd trimester
- May precipitate uric acid stones in patients with gout
Sodium acid phosphate
- History of gastrointestinal pain
- Sodium restriction
- History of kidney stones
- Risk of acute phosphate nephropathy
Pregnancy & Lactation
Pregnancy Category: C
Lactation: methenamine is excreted in human milk; caution advised, risk to infant cannot be ruled out
Pregnancy Categories
A: Generally acceptable. Controlled studies in pregnant women show no evidence of fetal risk.
B: May be acceptable. Either animal studies show no risk but human studies not available or animal studies showed minor risks and human studies done and showed no risk. C: Use with caution if benefits outweigh risks. Animal studies show risk and human studies not available or neither animal nor human studies done. D: Use in LIFE-THREATENING emergencies when no safer drug available. Positive evidence of human fetal risk. X: Do not use in pregnancy. Risks involved outweigh potential benefits. Safer alternatives exist. NA: Information not available.Pharmacology
Methenamine
Half-Life: 3-6 hr
Absorption: well absorbed; 10-30% hydrolyzed by gastric acid unless enteric coated tablet
Peak Urine Time: (formaldehyde) 2 hr for hippurate salt, 3-8 hr for mandelate salt
Metabolism: by liver (10-25%)
Excretion: urine (90%)
Sodium Acid Phosphate
Onset: 1-3 hr
Duration: 1-3 hr
Bioavailability: 66%
Excretion: urine: 90%
Mechanism of Action
Methenamine: Hydrolyzed in acidic urine to ammonia and formaldehyde, which are bactericidal agents; does not convert to formaldehyde in serum
Sodium acid phosphate: Urinary acidifier; elicits osmotic effect in GI tract
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