Mechanism of Action of Dicyclomine
Dicyclomine relieves gastrointestinal smooth muscle spasm through a dual mechanism: it acts as a specific anticholinergic (antimuscarinic) agent at acetylcholine receptor sites and has a direct effect on smooth muscle (musculotropic action). 1
Primary Mechanism
Dicyclomine works through two distinct mechanisms:
Anticholinergic (Antimuscarinic) Effect:
- Blocks acetylcholine receptors with approximately 1/8 the milligram potency of atropine
- Inhibits parasympathetic nerve impulses by blocking the binding of acetylcholine to muscarinic receptors
- This reduces gastrointestinal motility and secretions
Direct Smooth Muscle Effect (Musculotropic):
- Acts directly on smooth muscle independent of cholinergic receptors
- Antagonizes bradykinin and histamine-induced spasms
- This effect distinguishes it from pure anticholinergic agents like atropine
Pharmacodynamic Effects
Dicyclomine's pharmacodynamic effects include:
- Decreased gastrointestinal secretions and motility
- Inhibition of saliva and sweat secretion
- Pupil dilation
- Increased heart rate
- Depression of motor function 1
Comparative Potency
- In vivo studies show dicyclomine is equally potent against acetylcholine and barium chloride-induced intestinal spasm
- By contrast, atropine is at least 200 times more potent against acetylcholine than barium chloride
- Dicyclomine is approximately 1/500 as potent as atropine in mydriatic effects
- It is 1/300 as potent as atropine in antisialagogue effects 1
Clinical Applications
The British Society of Gastroenterology recognizes dicyclomine as an antimuscarinic antispasmodic that is effective for treating IBS symptoms, particularly:
- Abdominal pain
- Global symptom improvement
- Reducing intestinal motility 2
Pharmacokinetics
After oral administration:
- Rapidly absorbed, reaching peak values within 60-90 minutes
- Extensively distributed in tissues (mean volume of distribution ~3.65 L/kg)
- Primary excretion via urine (79.5% of dose)
- Secondary excretion via feces (8.4%)
- Initial plasma elimination half-life of approximately 1.8 hours
- Secondary phase of elimination with somewhat longer half-life 1
Tissue-Specific Effects
Laboratory studies demonstrate that dicyclomine's effects vary by tissue type:
- Most pronounced in bladder body tissue
- Moderate in bladder base tissue
- Minimal in proximal urethral tissue 3
This differential effect on various smooth muscle tissues explains why dicyclomine may provide targeted relief for certain gastrointestinal symptoms while causing fewer systemic anticholinergic effects than more potent agents like atropine.