Loperamide (Imodium)
Peripheral mu-opioid receptor agonist that reduces intestinal motility without crossing the blood-brain barrier at therapeutic doses — the P-gp efflux transporter actively pumps loperamide out of the CNS. This peripheral selectivity makes it the safest opioid for diarrhea with minimal abuse potential at standard doses. However, at supratherapeutic doses (60-800 mg — 15-200x normal), P-gp saturation and inhibition allows CNS penetration → euphoria and severe respiratory depression. P-gp inhibitors (quinidine, ritonavir, itraconazole) increase CNS effects. CYP3A4 and CYP2C8 substrate with extensive first-pass metabolism. FDA issued warnings about cardiac events (Torsades de Pointes) from hERG channel blockade at high doses.
Projected serum levels — 4 mg, as needed (shown daily)
Maintenance schedule: 4 mg as needed (shown daily) (oral).
Loading schedule: 5.3 mg on day 1, then 4 mg as needed (shown daily) — reaching therapeutic levels sooner.
Modeled steady state after ~2 days: peak ≈ 0.21 mg, trough ≈ 0.069 mg body load. Population-based estimate over 15 days for a 4 mg dose at a 70 kg reference body mass — the interactive app scales curves to your doses, timing, and body mass.
Key facts
- Category
- Antidiarrheal
- Route modeled
- Oral
- Model confidence
- inferred
- Half-life
- 11 h
- Common dose
- 4 mg
- Suggested maximum
- 16 mg/day
- Reference dose range
- 2–16 mg (single dose)
- Suggested cadence
- as needed (shown daily)
- Validated against
- 4 mg oral — Cmax 0.002 mg/L at 5 h
Documented interactions
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danger
Carbamazepine (Tegretol) + Loperamide (Imodium)
Carbamazepine (Tegretol) inducer of CYP3A4 predicted to change Loperamide (Imodium) AUC by ~0.33x
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danger
Efavirenz (Sustiva) + Loperamide (Imodium)
Efavirenz (Sustiva) inducer of CYP3A4 predicted to change Loperamide (Imodium) AUC by ~0.22x
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danger
Levothyroxine (Synthroid) + Loperamide (Imodium)
Loperamide (Imodium) may displace Levothyroxine (Synthroid) from plasma protein binding sites, transiently raising free drug concentration. This drug has a narrow therapeutic index; free…
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danger
Methadone + Loperamide (Imodium)
Loperamide (Imodium) and Methadone both push the delta opioid, herg, and mu opioid in the same direction.
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danger
Phenobarbital + Loperamide (Imodium)
Phenobarbital inducer of CYP3A4 predicted to change Loperamide (Imodium) AUC by ~0.50x
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danger
Phenytoin (Dilantin) + Loperamide (Imodium)
Phenytoin (Dilantin) inducer of CYP3A4 predicted to change Loperamide (Imodium) AUC by ~0.50x
Serum checks 283 modeled interaction pairings for loperamide (imodium) across your whole stack — absorption conflicts, enzyme inhibition, and nutrient depletion included.
Research behind this entry
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Loperamide: a pharmacological review
Comprehensive pharmacological review establishing loperamide as the preferred antidiarrheal due to P-gp-mediated peripheral mu-opioid selectivity, superior to diphenoxylate (which contains atropine to deter abuse), with…
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Loperamide misuse and abuse: a growing concern
Review documenting emerging loperamide abuse at supratherapeutic doses (70-800 mg/day) for opioid withdrawal self-treatment and euphoria, with FDA adverse event reports showing 48 cases of serious cardiac events…
2 published studies referenced in the app, each with a plain-language summary.