What is the mechanism of action of Bentyl (dicyclomine)?

Mechanism of Action of Bentyl (Dicyclomine)

Bentyl (dicyclomine) relieves gastrointestinal smooth muscle spasm through a dual mechanism: a specific anticholinergic (antimuscarinic) effect at acetylcholine receptor sites and a direct effect upon smooth muscle (musculotropic action). 1

Primary Mechanism of Action

• Dicyclomine functions as an antimuscarinic agent that binds to acetylcholine receptor sites with approximately 1/8 the milligram potency of atropine (based on in vitro studies using guinea pig ileum) 1
• It also has a direct musculotropic effect, demonstrated by its ability to antagonize bradykinin and histamine-induced spasms of isolated guinea pig ileum, an action not seen with atropine 1
• In vivo studies show dicyclomine is equally effective against acetylcholine-induced and barium chloride-induced intestinal spasm, while atropine is at least 200 times more potent against acetylcholine than barium chloride 1

Pharmacodynamic Effects

• Dicyclomine inhibits secretion of saliva and sweat 1
• Decreases gastrointestinal secretions and motility 1
• May cause drowsiness and dilate pupils 1
• Can increase heart rate 1
• Depresses motor function 1

Pharmacokinetics

• Rapidly absorbed after oral administration, reaching peak plasma concentrations within 60-90 minutes 1
• Mean volume of distribution for a 20 mg oral dose is approximately 3.65 L/kg, suggesting extensive tissue distribution 1
• Primary route of excretion is via urine (79.5% of dose) with some excretion in feces (8.4%) 1
• Mean plasma elimination half-life is approximately 1.8 hours initially, with a secondary phase of elimination showing a somewhat longer half-life 1

Clinical Application

• Dicyclomine is commonly used to reduce abdominal pain associated with irritable bowel syndrome (IBS) 2
• The American Gastroenterological Association (AGA) suggests using antispasmodics like dicyclomine in patients with IBS (conditional recommendation, low certainty in evidence) 2
• Antispasmodics with anticholinergic properties like dicyclomine appear to be more effective for pain relief than direct smooth muscle relaxants 3

Efficacy Considerations

• Meta-analyses show antispasmodics provide adequate global relief of IBS symptoms compared to placebo (RR, 0.67; 95% CI, 0.55–0.80) 2
• Antispasmodics also demonstrate improvement in abdominal pain compared to placebo (RR, 0.74; 95% CI, 0.59–0.93) 2
• Clinical trials have shown dicyclomine at doses of 40 mg four times daily to be superior to placebo in improving overall condition, decreasing abdominal pain and tenderness, and improving bowel habits 4

Common Side Effects and Limitations

• Most common adverse events include dry mouth, dizziness, and blurred vision due to anticholinergic effects 2
• Regular use in constipation-predominant conditions may be limited due to its anticholinergic effects 2
• Anticholinergic side effects may limit use in some patients 2

Clinical Considerations

• Dicyclomine is one of only three antispasmodics (along with hyoscine and peppermint oil) available in the United States for IBS treatment 2
• Individual response to dicyclomine varies between patients 3
• Anticholinergic effects are less potent than with atropine - dicyclomine is approximately 1/500 as potent as atropine for mydriatic effects and 1/300 as potent for antisialagogue effects 1