What are the side effects of bethanechol (muscarinic receptor agonist)?

Side Effects of Bethanechol

Bethanechol, as a muscarinic receptor agonist, causes predictable cholinergic side effects that mirror excessive acetylcholine stimulation, with the most common being gastrointestinal cramping, urinary urgency, salivation, sweating, and bronchial constriction. 1

Mechanism of Side Effects

Bethanechol produces adverse effects through direct muscarinic receptor activation throughout the body, similar to the effects seen with other cholinergic agents. 2 The side effects result from overstimulation of muscarinic receptors in smooth muscle, cardiac muscle, and secretory glands. 2

Common Side Effects (by System)

Gastrointestinal Effects

• Abdominal cramps or discomfort and colicky pain are among the most frequent complaints 1
• Nausea, belching, and diarrhea occur due to increased gastrointestinal motility 1
• Borborygmi (intestinal rumbling) and excessive salivation 1

Genitourinary Effects

• Urinary urgency is a predictable muscarinic effect 1
• This reflects the drug's intended therapeutic action on bladder smooth muscle 3

Cardiovascular Effects

• Fall in blood pressure with reflex tachycardia 1
• Vasomotor responses including flushing 1
• Bradycardia can occur through direct muscarinic stimulation, similar to other cholinergic agents 2

Respiratory Effects

• Bronchial constriction and potential asthmatic attacks in susceptible individuals 1
• This effect is particularly concerning in patients with underlying reactive airway disease 2

Dermatologic Effects

• Flushing producing a feeling of warmth 1
• Sensation of heat about the face 1
• Excessive sweating (perspiration) due to muscarinic effects on secretory glands 1, 2

Neurologic Effects

• Headache 1
• Malaise 1
• Seizures have been reported, though causal relationship remains uncertain 1

Special Senses

• Excessive lacrimation (tearing) 1
• Miosis (pupillary constriction) 1

Rare but Serious Adverse Effects

Acute Dystonic Reactions

• An acute dystonic reaction to bethanechol has been documented in at least one case, representing a rare but serious complication 4
• This occurred in an infant who also demonstrated dystonic reactions to dopamine receptor blocking agents 4

Important Clinical Considerations

Route-Dependent Risk

• Adverse reactions are rare following oral administration but more common with subcutaneous injection 1
• Side effects are more likely when dosage is increased 1

Comparison to Anticholinergic Toxicity

The side effect profile of bethanechol represents the opposite of anticholinergic toxicity. While anticholinergics cause hot, dry skin and urinary retention 5, bethanechol causes sweating and urinary urgency 1. This distinction is clinically important when bethanechol is used to counteract anticholinergic side effects of other medications. 6

Drug Specificity

Bethanechol displays the well-known behavioral effects of a direct muscarinic acetylcholine receptor agonist, distinguishing it from other gastrointestinal motility agents that work through different mechanisms. 7

Clinical Pitfalls to Avoid

• Do not use in patients with asthma or reactive airway disease due to risk of bronchospasm 1
• Monitor for cardiovascular effects, particularly hypotension and reflex tachycardia 1
• Be aware that the intensity of side effects correlates with dose escalation 1
• Recognize that oral administration carries lower risk than parenteral routes 1