MYOTONACHOL
Clinical safety rating
cautionComprehensive 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. |
| Molecular Weight | 162.19 Da |
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 | No adverse effects reported in animal studies; human data limited. Caution advised due to cholinergic effects. |
| 2nd trimester | No evidence of risk in second trimester; use only if clearly needed. |
| 3rd trimester | Cholinergic agents may induce uterine contractions; avoid near term unless for therapeutic induction. |
Clinical note
Comprehensive clinical and safety monograph for MYOTONACHOL (MYOTONACHOL).
| Placental transfer | Placental transfer occurs; extent not well quantified. Cholinergic agents can cross placenta and may affect fetal heart rate. |
| Breastfeeding | Excretion into breast milk is unknown. Due to potential for cholinergic effects in infant, caution is recommended. Use only if benefit outweighs risk. |
| Lactation Rating | L3 (Moderately Safe) |
| 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. |
| 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. |
■ FDA Black Box Warning
None
| Serious Effects |
Hypersensitivity to bethanechol or any componentMechanical obstruction of GI or urinary tractAsthmaHyperthyroidismPeptic ulcer diseaseEpilepsyParkinsonismCoronary artery diseaseBradycardiaHypotensionVasomotor instability
| 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). |
| Food/Dietary | Food decreases absorption; take on an empty stomach. Avoid high-fat meals as they may increase side effects. No known specific food interactions. |
| Clinical Pearls | MYOTONACHOL (bethanechol) is a cholinergic agonist used for urinary retention. Monitor for bradycardia and bronchospasm, especially in patients with asthma or cardiac disease. Administer on an empty stomach to reduce nausea. Avoid use in patients with GI obstruction or recent bladder surgery. Atropine should be readily available as an antidote. |
| Patient Advice | Take this medication on an empty stomach, 1 hour before or 2 hours after meals. · Avoid alcohol and caffeine, as they may worsen side effects. · Report symptoms like slow heart rate, wheezing, dizziness, or excessive sweating. · Do not drive or operate heavy machinery until you know how this drug affects you. · If you miss a dose, skip it and take the next dose at the regular time; do not double the dose. |
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