Dosing & Uses
Dosage Forms & Strengths
methenamine/hyoscyamine
tablet
- 81mg/0.12mg
tablet, capsule
- 120mg/0.12mg
Urinary Tract Irritative Voiding Symptoms
1 tablet/capsule PO q6hr with liberal fluid intake
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
Hyoscyamine: Adjunctive treatment for lower urinary tract hypermotility and spasm
Dosage Forms & Strengths
tablet
- 81 mg/0.12mg
tablet, capsule
- 120 mg/0.12mg
Urinary Tract Infection Prophylaxis
<16 years: Safety/efficacy not established
≥16 years: 1 tablet/capsule PO q6hr with liberal fluid intake
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
Hyoscyamine: Adjunctive treatment for lower urinary tract hypermotility and spasm
Avoid hyoscyamine except in short-term situations to decrease secretions; high incidence of anticholinergic effects (Beers criteria)
Interactions
Interaction Checker
No Results

Contraindicated
Serious - Use Alternative
Significant - Monitor Closely
Minor

Adverse Effects
>10%
Hyoscyamine
- Dry skin
1-10%
Methenamine
- Dysuria
- Discoloration of urine (blue)
- Gastric upset
- Nausea
- Rash
Hyoscyamine
- Blurred vision
- Constipation
- Dysphagia
- Photosensitivity
<1%
Hyoscyamine
- Headache
- Orthostatic hypotension
- Tachycardia
Warnings
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
Hyoscyamine
- Hypersensitivity to hyoscyamine or related compounds
- Closed-angle glaucoma, myasthenia gravis, hemorrhage w/ cardiovascular instability, paralytic ileus, intestinal atony of elderly/debilitated pt, obstructive uropathy, toxic megacolon, GI obstruction, tachycardia secondary to cardiac insufficiency or thyrotoxicosis
- Breastfeeding
Cautions
Methenamine
- Large doses may cause bladder irritation, urinary frequency, albuminuria, and hematuria
- Maintain acidic pH of urine, esp. 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
Hyoscyamine
- Renal/hepatic impairment, BPH, CHF, CAD, HTN, COPD, hiatal hernia, reflux esophagitis, mitral stenosis, brain damage or spastic paralysis in children, salivary secretion disorder, Down syndrome, autonomic neuropathy, hyperthyroidism, tachyarrythmia, toxin-mediated diarrhea
- Elderly
Pregnancy & Lactation
Pregnancy Category: C
Lactation: hyoscyamine and methenamine excreted in human milk; caution in breastfeeding women
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
Mechanism of Action
Methenamine: Hydrolyzed in acidic urine to ammonia and formaldehyde, which are bactericidal agents; does not convert to formaldehyde in serum
Hyoscyamine: Elicits anticholinergic activity; decreases urinary spasms with cystitis
Additional ingredients in some products: Methylene blue (weak antiseptic properties); sodium phosphate monobasic (acidifier); phenyl salicylate (mild analgesic)
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
Liver (10-25%)
Elimination
Half-Life: 3-6 hr
Excretion: urine (90%)