Bethanechol is a classic and highly testable drug in autonomic pharmacology and urology. It represents the opposite approach to antimuscarinic drugs used in overactive bladder. While drugs like oxybutynin reduce bladder contractions, bethanechol stimulates bladder emptying.
For medical, nursing, pharmacy, and allied health students, bethanechol is important because it clearly demonstrates:
- Parasympathomimetic drug action
- Muscarinic receptor physiology (M₃ receptors)
- Clinical management of urinary retention
What Is Bethanechol?
Bethanechol is a direct-acting parasympathomimetic (cholinergic) drug that selectively stimulates muscarinic receptors.
It primarily acts on:
1. Urinary bladderIts main clinical use is to promote bladder emptying in patients with urinary retention.
Pharmacological Classification
Drug Class
- Direct-acting cholinergic agonist
- Muscarinic receptor agonist
Therapeutic Category
- Treatment of urinary retention
- GI motility stimulant (limited use)
Mechanism of Action (Very High-Yield)
This is the most important section for exams.
How Bethanechol Works
Bethanechol:
- Directly stimulates M₃ muscarinic receptors
- Mimics the action of acetylcholine
- Has no nicotinic activity
Effects on the Urinary Bladder
- Contracts the detrusor muscle
- Relaxes the bladder trigone and sphincter
- Promotes micturition
Net Effect
- Increased bladder tone
- Improved bladder emptying
- Relief of urinary retention
Exam Pearl:
Bethanechol is resistant to acetylcholinesterase → longer duration of action.
Why Bethanechol Is Bladder-Selective
Bethanechol has two important properties:
1. Selective muscarinic action (no nicotinic effects)This makes it suitable for therapeutic use, unlike acetylcholine itself.
Indications of Bethanechol
Primary Indications (Must Remember)
1. Postoperative urinary retentionOther Uses (Less Common)
Gastrointestinal atony (postoperative ileus)Conditions Where Bethanechol Is NOT Useful
- ❌ Mechanical obstruction of urinary tract
- ❌ Prostatic enlargement causing obstruction
- ❌ Overactive bladder
Bethanechol works only when the problem is poor detrusor contraction, not obstruction.
Routes of Administration
| Route | Use |
|---|---|
| Oral | Most common |
| Subcutaneous | Rare, controlled settings |
Important: Bethanechol is not given intravenously.
Pharmacological Effects (System-Wise)
Urinary System
- Increased detrusor contraction
- Facilitates urination
Gastrointestinal System
- Increased peristalsis
- Increased gastric tone
Cardiovascular System
- Bradycardia
- Hypotension (rare at therapeutic doses)
Respiratory System
Bronchoconstriction (important precaution)Adverse Effects and Side Effects
Most side effects are due to excess muscarinic stimulation.
Common Side Effects
- Abdominal cramps
- Diarrhea
- Increased salivation
- Sweating
Cardiovascular
- Bradycardia
- Hypotension
Respiratory
Bronchospasm (important in asthmatics)Others
- Flushing
- Lacrimation
- Urinary urgency
Mnemonic for side effects:
DUMBBELSS (Diarrhea, Urination, Miosis, Bronchospasm, Bradycardia, Emesis, Lacrimation, Sweating, Salivation)
Contraindications (Very Important for Exams)
Bethanechol is contraindicated in:
- Asthma or COPD
- Peptic ulcer disease
- Coronary artery disease
- Bradycardia or hypotension
- Mechanical obstruction of GI or urinary tract
Drug Interactions
- Additive effects with other cholinergic drugs
- Antagonized by antimuscarinic drugs (e.g., atropine)
Nursing & Patient Education (Exam-Friendly)
Before Administration
- Rule out mechanical obstruction
- Assess bladder distension
During Therapy
- Monitor heart rate and blood pressure
- Observe for signs of cholinergic excess
Patient Counseling
- Take medication as prescribed
- Report breathing difficulty or severe abdominal pain
- Expect increased urge to urinate
Comparison: Bethanechol vs Antimuscarinic Drugs
| Feature | Bethanechol | Oxybutynin / Tolterodine |
|---|---|---|
| Action | Stimulates bladder contraction | Reduces bladder contraction |
| Receptor | Muscarinic agonist (M₃) | Muscarinic antagonist |
| Use | Urinary retention | Overactive bladder |
| Effect on micturition | Increases | Decreases |
Clinical & Exam Pearls (Must Remember)
- Bethanechol is a direct muscarinic agonist
- Used in urinary retention, not incontinence
- Acts mainly on M₃ receptors
- Does not act on nicotinic receptors
- Avoid in asthma and cardiac disease
Summary Table: Bethanechol at a Glance
| Parameter | Key Point |
|---|---|
| Drug class | Direct cholinergic agonist |
| Main receptor | M₃ muscarinic |
| Main use | Urinary retention |
| Effect on bladder | Contracts detrusor |
| Key risk | Bronchospasm |
| Exam relevance | Very high |
FAQs
1. Why is bethanechol used in urinary retention?
It stimulates detrusor muscle contraction via M₃ receptors.
2. Can bethanechol be used in overactive bladder?
No. It worsens symptoms by increasing bladder contractions.
3. Does behanechol act on nicotinic receptors?
No. It is selective for muscarinic receptors.
4. Why is bethanechol not degraded by acetylcholinesterase?
Its chemical structure makes it resistant, prolonging its action.
5. Is bethanechol useful in prostatic obstruction?
No. Mechanical obstruction must be relieved first.
6. Why is bethanechol contraindicated in asthma?
It can cause bronchoconstriction due to muscarinic stimulation.
7. What is the most common side effect?
Abdominal cramps and diarrhea.
8. Can bethanechol cause hypotension?
Yes, due to vasodilation and bradycardia.
9. What drug can reverse bethanechol toxicity?
Atropine (antimuscarinic).
10. What is the most tested exam point?
Bethanechol is used for urinary retention by stimulating M₃ receptors.
