Hyoscyamine - Drug Monograph

Introduction

Hyoscyamine is an anticholinergic medication derived from plants of the Solanaceae family, including deadly nightshade (Atropa belladonna), henbane, and jimsonweed. It is a naturally occurring tropane alkaloid that represents the levo-isomer of atropine. Hyoscyamine is clinically used for its parasympatholytic effects on smooth muscle, secretory glands, and the central nervous system.

Mechanism of Action

Hyoscyamine competitively inhibits muscarinic acetylcholine receptors, particularly M1, M2, and M3 subtypes. By blocking the action of acetylcholine at postganglionic parasympathetic neuroeffector sites, it reduces smooth muscle spasms in the gastrointestinal, biliary, and urinary tracts; decreases secretory activity (salivary, gastric, pancreatic, and bronchial); and reduces rigidity and tremors associated with Parkinsonism. Its central effects are due to penetration through the blood-brain barrier.

Indications

• Gastrointestinal disorders: Irritable bowel syndrome, diverticulitis, colitis
• Genitourinary conditions: Renal colic, cystitis, ureteral spasms
• Biliary disorders: Cholecystitis, cholelithiasis
• Reducing secretions: Pre-anesthetic medication to reduce salivary and respiratory secretions
• Parkinson's disease: Adjunctive therapy for reducing rigidity and tremors
• Motion sickness: Prevention and treatment

Dosage and Administration

• Oral: 0.125-0.25 mg every 4-6 hours as needed (maximum 1.5 mg/day)
• Sublingual: 0.125-0.25 mg dissolved under tongue 3-4 times daily
• Extended-release: 0.375 mg every 12 hours

• 0.0625-0.125 mg every 4 hours (3-4 times daily)
• Maximum dose: 0.75 mg/24 hours

• Renal impairment: Dose reduction required (CrCl <30 mL/min)
• Hepatic impairment: Use with caution
• Geriatric: Lower doses recommended due to increased sensitivity

Pharmacokinetics

Contraindications

• Hypersensitivity to hyoscyamine or related compounds
• Glaucoma (narrow-angle)
• Obstructive uropathy (bladder neck obstruction)
• Gastrointestinal obstruction
• Paralytic ileus
• Intestinal atony
• Severe ulcerative colitis
• Toxic megacolon
• Myasthenia gravis
• Unstable cardiovascular status

Warnings and Precautions

• Use with caution in patients with:

- Autonomic neuropathy - Hepatic or renal impairment - Hyperthyroidism - Coronary artery disease - Congestive heart failure - Cardiac arrhythmias - Hypertension - Hiatal hernia with reflux esophagitis

• May cause drowsiness or blurred vision
• Risk of heat prostration in hot environments
• May produce anhidrosis
• Use in pregnancy (Category C) only if potential benefit justifies risk
• Excreted in breast milk - use caution during breastfeeding

Drug Interactions

• Other anticholinergics: Additive effects (tricyclic antidepressants, phenothiazines, antihistamines)
• Amantadine: Increased anticholinergic effects
• Digoxin: Increased serum digoxin concentrations
• Ketoconazole: Reduced absorption of ketoconazole
• Metoclopramide: Antagonizes effect
• Potassium chloride: Increased risk of GI mucosal lesions
• CNS depressants: Additive sedative effects

Adverse Effects

• Dry mouth
• Blurred vision
• Photophobia
• Constipation
• Urinary retention
• Tachycardia
• Drowsiness
• Dizziness

• Angle-closure glaucoma
• Paralytic ileus
• Hyperthermia
• Heat stroke
• Cardiac arrhythmias
• Confusion (especially elderly)
• Hallucinations
• Anaphylaxis

Monitoring Parameters

• Heart rate and rhythm
• Blood pressure
• Intraocular pressure (baseline and periodic)
• Bowel sounds and function
• Urinary output
• Mental status (especially in elderly)
• Signs of overdose: flushing, hyperthermia, tachycardia

Patient Education

• Take exactly as prescribed; do not exceed recommended dose
• Allow sublingual tablets to dissolve completely under tongue
• Swallow extended-release tablets whole; do not crush or chew
• May cause drowsiness or blurred vision - avoid driving or operating machinery
• Maintain adequate fluid intake to prevent constipation
• Use sugar-free candy or gum for dry mouth relief
• Avoid alcohol and other CNS depressants
• Report immediately: rapid heartbeat, difficulty urinating, eye pain, or confusion
• Store at room temperature away from moisture

References

1. American Society of Health-System Pharmacists. AHFS Drug Information. Hyoscyamine monograph. 2023. 2. Brunton LL, Hilal-Dandan R, Knollmann BC. Goodman & Gilman's: The Pharmacological Basis of Therapeutics. 13th ed. McGraw-Hill Education; 2017. 3. Lexicomp Online. Hyoscyamine monograph. Wolters Kluwer Clinical Drug Information; 2023. 4. National Institutes of Health. DailyMed. Hyoscyamine sulfate labeling. 2022. 5. Thomsen HS, Muller RN, Mattrey RF. Trends in Pharmacological Sciences. 2004;25(5):247-253. 6. Clinical Pharmacology [database online]. Hyoscyamine. Elsevier; 2023.