Lomotil
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Here is a comprehensive overview of Lomotil (diphenoxylate/atropine):
Lomotil (Diphenoxylate + Atropine)
What It Is
Lomotil is a combination antidiarrheal agent containing diphenoxylate 2.5 mg + atropine 0.025 mg per tablet (or per 5 mL liquid). It is a Schedule V controlled substance.
Mechanism of Action
Diphenoxylate is a meperidine analog that acts as a mu-opioid receptor (MOR) agonist in the enteric nervous system. It:
- Activates presynaptic opioid receptors → inhibits acetylcholine release
- Decreases peristalsis and intestinal motility
- Reduces fluid and electrolyte loss through the gut
It acts peripherally at usual doses and has no significant analgesic effect. Diphenoxylate is rapidly de-esterified to its active metabolite difenoxin, which has a half-life of ~12 hours.
The atropine component (at subtherapeutic doses) is added purely as a deterrent to abuse — high-dose misuse produces unpleasant anticholinergic effects (dry mouth, blurred vision, tachycardia, nausea).
Indications
- Acute diarrhea (adjunct therapy)
- Chronic diarrhea (e.g., short bowel syndrome, postvagotomy diarrhea)
- Traveler's diarrhea (though loperamide is generally preferred)
Dosing (Adults)
| Form | Initial Dose | Maintenance | Maximum |
|---|---|---|---|
| Tablets (2.5 mg diphenoxylate) | 2 tablets (5 mg) | 1 tablet q3–4h | 20 mg/day |
| Liquid (2.5 mg/5 mL) | 10 mL | per response | 20 mg/day |
- If no improvement in 48 hours for acute diarrhea → discontinue
- If no improvement in 10 days for chronic diarrhea at max dose → unlikely to be effective
- Once controlled, taper dose down
Pediatric dose: 0.3–0.4 mg/kg/day in 4 divided doses (max 10 mg/day); use with caution.
Contraindications & Precautions
| Condition | Reason |
|---|---|
| Active ulcerative colitis / inflammatory bowel disease involving colon | Risk of toxic megacolon |
| Severe infectious diarrhea (fever, bloody stool) | Antimotility effect may retain pathogens, prolonging illness |
| Young children | Risk of respiratory depression, coma, cardiac events, death |
| Obstructive jaundice | Contraindicated |
Adverse Effects
- Usual doses: Constipation, abdominal cramping, nausea
- High doses (40–60 mg/day): CNS opioid effects (sedation, euphoria, respiratory depression)
- Overdose/abuse: CNS depression + anticholinergic toxidrome from atropine — mydriasis, dry mouth, urinary retention, tachycardia, blurred vision
- Toxic megacolon in inflammatory bowel disease
Overdose / Toxicology
Lomotil overdose is particularly dangerous because:
- Onset of toxicity may be delayed up to 12 hours after ingestion (due to the pharmacokinetics of diphenoxylate → difenoxin metabolism)
- Asymptomatic patients with known/suspected Lomotil ingestion must be observed in a monitored setting
- The clinical picture combines opioid toxidrome (miosis, respiratory depression, CNS depression) plus anticholinergic signs (mydriasis may partially mask miosis)
- Treatment follows standard opioid overdose management (naloxone)
— Tintinalli's Emergency Medicine; Rosen's Emergency Medicine; Harrison's Principles of Internal Medicine 22E
Comparison with Loperamide (Imodium)
| Feature | Diphenoxylate (Lomotil) | Loperamide (Imodium) |
|---|---|---|
| CNS penetration | Yes (at high doses) | Minimal |
| Abuse potential | Yes (Schedule V) | Lower (OTC) |
| Atropine added | Yes (deterrent) | No |
| Efficacy | Slightly less effective | More effective |
| Availability | Prescription | OTC |
Loperamide is generally more effective and preferred for general use; diphenoxylate is reserved for cases where loperamide is inadequate or a prescription agent is preferred.
— Lippincott Illustrated Reviews: Pharmacology; Goodman & Gilman's Pharmacological Basis of Therapeutics
Loperamide
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Here is a comprehensive overview of Loperamide (Imodium):
Loperamide (Imodium / Imodium A-D)
What It Is
Loperamide is an OTC antidiarrheal and a meperidine analog that acts primarily in the gut. It is generally preferred over diphenoxylate (Lomotil) for most diarrheal conditions due to greater efficacy, wider availability, and a more favorable safety profile at therapeutic doses.
Mechanism of Action
Loperamide is a mu-opioid receptor (MOR) agonist that acts on the myenteric plexus of the enteric nervous system:
- Reduces smooth muscle tone and decreases intestinal motility
- Increases mouth-to-cecum and small intestinal transit times
- Increases anal sphincter tone
- Has antisecretory activity — counters cholera toxin and some E. coli toxins by acting on G-protein–linked receptors to oppose adenylyl cyclase stimulation
At therapeutic doses, loperamide is 40–50× more potent than morphine as an antidiarrheal, yet CNS penetration is minimal because it is actively pumped out of the CNS by P-glycoprotein (P-gp) efflux transporters at the blood-brain barrier.
Pharmacokinetics (ADME)
| Parameter | Value |
|---|---|
| Route | Oral (capsule, solution, chewable tablet) |
| Peak plasma levels | 3–5 hours |
| Half-life (t½) | ~11 hours |
| Metabolism | Extensive hepatic |
| CNS penetration | Minimal (P-gp efflux) |
Indications
- Acute diarrhea (including traveler's diarrhea — alone or with antibiotics)
- Chronic diarrhea (IBD adjunct, short bowel syndrome, functional diarrhea)
- Pregnancy: preferred antidiarrheal (peripherally acting, not known to be teratogenic, though associated with lower birth weights)
Dosing
Adults:
| Indication | Initial Dose | Subsequent | Maximum |
|---|---|---|---|
| Acute diarrhea | 4 mg | 2 mg after each loose stool | 16 mg/day |
| Chronic diarrhea | 4 mg initially | 4–8 mg/day in divided doses | 16 mg/day |
- Discontinue if no improvement in 48 hours (acute) or if not responding
- Adding simethicone 60–125 mg up to 4×/day can improve symptom relief (gas/cramping)
Pediatric maximum daily doses:
| Age | Max Daily Dose |
|---|---|
| 2–5 years | 3 mg |
| 6–8 years | 4 mg |
| 8–12 years | 6 mg |
| < 2 years | Not recommended |
Contraindications & Precautions
| Condition | Reason |
|---|---|
| Active IBD (colitis) involving the colon | Risk of toxic megacolon — use with great caution or avoid |
| Infectious diarrhea with fever / blood in stool | Antimotility effect retains pathogens |
| Children < 2 years | Risk of respiratory depression, CNS depression |
Adverse Effects (Therapeutic Doses)
- Constipation
- Abdominal cramping / bloating
- Nausea
- Paralytic ileus (overdose)
Overdose / Abuse — Important Safety Concern
Loperamide has seen rising rates of misuse ("the poor man's methadone") since 2010. At supratherapeutic doses (50–300 mg; 20–150× the 2 mg therapeutic dose):
- Overcomes P-gp efflux → penetrates the CNS → opioid euphoria and CNS/respiratory depression
- Blocks sodium and potassium channels → severe cardiotoxicity:
- QT prolongation
- QRS widening
- Ventricular tachycardia / fibrillation
- Torsades de pointes
- Cardiac arrest, death
FDA black box warning: Exceeding recommended dosage can cause fatal cardiac events.
Drug Interaction Risk
Co-ingestion with P-gp inhibitors (e.g., quinidine, ritonavir, some azole antifungals) dramatically increases loperamide bioavailability and CNS penetration even at lower doses → life-threatening cardiovascular toxicity.
Treatment of Overdose
| Target | Intervention |
|---|---|
| GI decontamination | Activated charcoal (can repeat — loperamide inhibits peristalsis) |
| Respiratory depression | Naloxone 0.04–0.4 mg IV (standard doses) |
| Widened QRS / Na+ channel blockade | Sodium bicarbonate 1–2 mEq/kg IV + K⁺/Mg²⁺ replacement |
| Torsades de pointes | Cardiac overdrive pacing, isoproterenol 2–10 mcg/min IV |
| Refractory cases | IV lipid emulsion, ECMO (limited case evidence) |
Loperamide vs. Diphenoxylate (Lomotil) — Quick Comparison
| Feature | Loperamide | Diphenoxylate (Lomotil) |
|---|---|---|
| Efficacy | More effective | Slightly less |
| CNS penetration (therapeutic) | Minimal | Low (but more than loperamide) |
| Schedule | OTC | Schedule V (Rx) |
| Atropine combined | No | Yes (abuse deterrent) |
| Cardiac toxicity risk | Yes (at high doses) | Less prominent |
| Preferred in pregnancy | Yes | No (teratogenic in animals) |
— Goodman & Gilman's Pharmacological Basis of Therapeutics; Lippincott Illustrated Reviews: Pharmacology; Tintinalli's Emergency Medicine
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