Chemistry - A synthetic cholinergic ester, bethanechol occurs as a slightly hygroscopic, white or colorless crystalline powder with a slight, amine-like or “fishy” odor. It exhibits polymorphism, with one form melting at 211° and the other form at 219°. One gram of the drug is soluble in approximately 1 ml of water or 10 ml of alcohol. The commercially available injection has a pH from 5.5 - 7.5.
Storage/Stability/Compatibility - Bethanechol tablets should be stored at room temperature in tight containers. The injectable form should be stored at room temperature; avoid freezing. It may be autoclaved at 120°C for 20 minutes without any loss of potency.
Pharmacology - Bethanechol directly stimulates cholinergic receptors. Its effects are principally muscarinic and at usual doses has negligible nicotinic activity. It is more resistant to hydrolysis than acetylcholine by cholinesterase and, therefore, has an increased duration of activity.
Pharmacologic effects include increased esophageal peristalsis and lower esophageal sphincter tone, increased tone and peristaltic activity of the stomach and intestines, increased gastric and pancreatic secretions, increased tone of the detrusor muscle of the bladder, and decreased bladder capacity. At high doses after parenteral administration, effects such as increased bronchial secretions and constriction, miosis, lacrimation, and salivation can be seen. When administered SQ or orally, effects are predominantly on the GI and urinary tracts.
Uses/Indications - In veterinary medicine, bethanechol is used primarily to stimulate bladder contractions in small animals. It also can be used as an esophageal or general GI stimulant, but metoclopramide and/or neostigmine have largely supplanted it for these uses.
Pharmacokinetics - No information was located on the pharmacokinetics of this agent in veterinary species. In humans, bethanechol is poorly absorbed from the GI tract, and the onset of action is usually within 30-90 minutes after oral dosing. After subcutaneous administration, effects begin within 5-15 minutes and usually peak within 30 minutes. The duration of action after oral dosing may persist for up to 6 hours after large doses and 2 hours after SQ dosing. Subcutaneous administration yields a more enhanced effect on urinary tract stimulation than does oral administration.
Bethanechol does not enter the CNS after usual doses; other distribution aspects of the drug are not known. The metabolic or excretory fate of bethanechol have not been described.
Contraindications/Precautions - Contraindications to bethanechol therapy include: bladder neck or other urinary outflow obstruction, when the integrity of the bladder wall is in question (e.g., as after recent bladder surgery), hyperthyroidism, peptic ulcer disease or when other inflammatory GI lesions are present, recent GI surgery with resections/anastomoses, GI obstruction or peritonitis, hypersensitivity to the drug, epilepsy, asthma, coronary artery disease or occlusion, hypotension, severe bradycardia or vagotonia or vasomotor instability. If urinary outflow resistance is increased due to enhanced urethral tone (not mechanical obstruction!), bethanechol should only be used in conjunction with another agent that will sufficiently reduce outflow resistance (e.g., diazepam, dantrolene (striated muscle) or phenoxybenzamine (smooth muscle)).
Adverse Effects/Warnings - When administered orally to small animals, adverse effects are usually mild, with vomiting, diarrhea, salivation, and anorexia being the most likely to occur. Cardiovascular (arrhythmias, hypotension) and respiratory effects (asthma) are most likely only seen after overdosage situations or with high dose SQ therapy.
IM or IV use is not recommended, except in emergency situations when the IV route may be used. Severe cholinergic reactions are likely if given IV. If injecting the drug (SQ or IV), it is recommended that atropine be immediately available.
Overdosage - Symptoms of overdosage are basically cholinergic in nature. Muscarinic effects (salivation, urination, defecation, etc.) are usually seen with oral or SQ administration. If given IM or IV, a full-blown cholinergic crisis can occur with circulatory collapse, bloody diarrhea, shock and cardiac arrest possible.
Treatment for bethanechol toxicity is atropine. Refer to the atropine monograph for more information on its use. Epinephrine may also be employed to treat symptoms of bronchospasm.
Drug Interactions - Bethanechol should not be used concomitantly with other cholinergic (e.g., carbachol) or anticholinesterase (e.g., neostigmine) agents because of additive effects and increased likelihood of toxicity developing. Quinidine, procainamide, epinephrine (or sympathomimetic amines) or atropine can antagonize the effects of bethanechol. Bethanechol used in combination with ganglionic blocking drugs (e.g., mecamylamine) can produce severe GI and hypotensive effects.
a) 0.05 mg/kg SQ; 0.11 - 0.22 mg/kg IV; Start at lower dose first and use cautiously (McConnell and Hughey 1987)
Monitoring Parameters - Clinical efficacy; urination frequency, amount voided, bladder palpation; Adverse effects (see above section)
Client Information - Give medication on an empty stomach unless otherwise instructed by veterinarian. Contact veterinarian if salivation or GI (vomiting, diarrhea, or anorexia) effects are pronounced or persist.
Dosage Forms/Preparations/FDA Approval Status/Withholding Times -
Veterinary-Approved Products: None
Bethanechol Chloride Tablets 5 mg, 10 mg, 25 mg, 50 mg; Urecholine® (Merck, Frosst); Duvoid® (Roberts); Myotonachol® (Glenwood); PMS-Bethanechol Chloride® (Glenwood); Generic; (Rx)
Bethanechol Chloride for Injection 5 mg/ml in 1 ml vials and amps; Urecholine® (Frosst); (Rx)
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