Dosing & Uses
Dosage Forms & Strengths
methenamine/sodium acid phosphate
- 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
Serious - Use Alternative
Significant - Monitor Closely
Frequency Not Defined
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
- 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
- Addison's disease
- Acidification of urine in urinary stone disease
- Urolithiasis or struvite stone formation
- Severe renal impairment (ie, CrCl <30 mL/min)
- 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
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.
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
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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The above information is provided for general informational and educational purposes only. Individual plans may vary and formulary information changes. Contact the applicable plan provider for the most current information.
|1||This drug is available at the lowest co-pay. Most commonly, these are generic drugs.|
|2||This drug is available at a middle level co-pay. Most commonly, these are "preferred" (on formulary) brand drugs.|
|3||This drug is available at a higher level co-pay. Most commonly, these are "non-preferred" brand drugs.|
|4||This drug is available at a higher level co-pay. Most commonly, these are "non-preferred" brand drugs or specialty prescription products.|
|5||This drug is available at a higher level co-pay. Most commonly, these are "non-preferred" brand drugs or specialty prescription products.|
|6||This drug is available at a higher level co-pay. Most commonly, these are "non-preferred" brand drugs or specialty prescription products.|
|NC||NOT COVERED – Drugs that are not covered by the plan.|
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Drugs that have restrictions other than prior authorization, quantity limits, and step therapy associated with each prescription.