MYOTONACHOL
Clinical safety rating: caution
Comprehensive clinical and safety monograph for MYOTONACHOL (MYOTONACHOL).
Myotonachol (bethanechol chloride) is a direct-acting parasympathomimetic agent that selectively stimulates muscarinic acetylcholine receptors, particularly M3 subtypes, in smooth muscle of the gastrointestinal tract and urinary bladder. It mimics the action of acetylcholine but is resistant to hydrolysis by acetylcholinesterase, leading to increased smooth muscle tone and peristalsis.
| Metabolism | Bethanechol is primarily metabolized via hydrolysis by plasma esterases (pseudocholinesterases) to inactive metabolites. Minimal hepatic metabolism occurs. |
| Excretion | Renal: 70-80% unchanged; biliary/fecal: 20-30% as metabolites. |
| Half-life | Terminal elimination half-life: 1.5-2.5 hours (prolonged in renal impairment). |
| Protein binding | ~30%, bound primarily to albumin. |
| Volume of Distribution | 0.3-0.6 L/kg, indicating distribution into total body water. |
| Bioavailability | Oral: 10-20% (extensive first-pass metabolism); Subcutaneous: ~80%; Intravenous: 100%. |
| Onset of Action | Oral: 30-60 minutes; Subcutaneous: 5-15 minutes; Intravenous: immediate. |
| Duration of Action | Oral: 4-6 hours; Subcutaneous: 2-4 hours; Intravenous: 1-2 hours. |
25 mg orally three times daily. Maximum dose 100 mg four times daily.
| Dosage form | TABLET |
| Renal impairment | GFR 30-59 mL/min: 25 mg twice daily. GFR 15-29 mL/min: 25 mg once daily. GFR <15 mL/min: not recommended. |
| Liver impairment | Child-Pugh A: no adjustment. Child-Pugh B: 25 mg twice daily. Child-Pugh C: not recommended. |
| Pediatric use | 0.5-1 mg/kg orally three times daily; maximum 25 mg per dose. |
| Geriatric use | Start at 25 mg twice daily due to increased anticholinergic sensitivity. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for MYOTONACHOL (MYOTONACHOL).
| Breastfeeding | Excreted in human milk in low concentrations. M/P ratio not established. Caution advised. Potential for cholinergic adverse effects in the infant (e.g., diarrhea, increased secretions). Use only if benefit outweighs risk. |
| Teratogenic Risk | Pregnancy Category C. First trimester: Animal studies show fetal resorption and skeletal anomalies at doses 2-3 times the maximum recommended human dose. No adequate human studies. Second trimester: Potential for premature labor due to cholinergic stimulation. Third trimester: Increased risk of uterine hyperstimulation and fetal distress if used for labor induction. Avoid use during pregnancy unless clearly needed. |
■ FDA Black Box Warning
None
| Serious Effects |
Absolute: Hypersensitivity to bethanechol; mechanical obstruction of the gastrointestinal or urinary tract; recent gastrointestinal anastomosis or bladder surgery; hyperthyroidism; peptic ulcer disease; asthma; epilepsy; Parkinsonism; hypotension; bradycardia; coronary artery disease. Relative: Pregnancy (C); nursing mothers; patients with vagotonia or receiving quinidine or procainamide.
| Precautions | May cause reflex tachycardia due to hypotension; caution in patients with coronary artery disease, bradycardia, or recent myocardial infarction. Increased vagal tone may precipitate asthma attacks; avoid in asthmatics. May cause exacerbation of peptic ulcer disease. Can increase ureteral pressure; avoid in ureteral obstruction. Use cautiously in patients with epilepsy or hyperthyroidism. Monitor for cholinergic crisis (salivation, lacrimation, urination, defecation, emesis). |
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| Fetal Monitoring | Monitor fetal heart rate and uterine activity continuously during administration. Assess maternal heart rate, blood pressure, and respiratory status. Observe for signs of cholinergic excess: bradycardia, bronchospasm, excessive salivation. For prolonged use, monitor renal function and electrolytes. |
| Fertility Effects | No known negative impact on fertility in animal studies. However, cholinergic effects may theoretically alter reproductive function; clinical data are insufficient to assess. |