methenamine/hyoscyamine (Discontinued)

>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

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 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.

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%)