Enterohepatic circulation
The recycling of drugs or their metabolites from bile → intestine → portal vein → liver — extending effective half-life and creating secondary plasma peaks.
The EHC cycle
Conjugation (glucuronidation)
Biliary excretion via MRP2/BSEP
Bacterial β-glucuronidase cleaves conjugate
Active drug reabsorbed (lipophilic)
Drug returns to liver for re-conjugation
or systemic recirculation
Interruption points: cholestyramine binds drug in gut → blocks reabsorption → accelerates net elimination.
Drugs with clinically relevant EHC
| Drug | EHC contribution | Clinical consequence | Interruption approach |
|---|---|---|---|
| Leflunomide ↗ | Teriflunomide (active metabolite) extensively recycled; t½ ~2 weeks | Drug detectable for up to 2 years after discontinuation without washout | Cholestyramine 8 g TID × 11 days (or activated charcoal) |
| Ethinylestradiol (OC) | Glucuronide conjugate deconjugated by gut flora; aglycone reabsorbed | Broad-spectrum antibiotics may reduce OC plasma levels (modest effect) | N/A; use barrier contraception during antibiotic course (precautionary) |
| Digoxin ↗ | Minor biliary excretion (~6–10%); partial EHC | Secondary plasma peaks; cholestyramine reduces digoxin AUC | Separate cholestyramine dosing ≥2 h from digoxin |
| Morphine / M6G | M6G (active) undergoes partial biliary excretion and reabsorption | Prolonged opioid effect; M6G accumulation in renal impairment exacerbated | No specific intervention; renal dose adjustment essential |
| Atorvastatin / rosuvastatin | Biliary excretion of parent drug and metabolites; partial EHC | OATP1B1/BCRP interactions affect hepatic uptake; cholestyramine co-administration reduces statin levels | Separate bile acid sequestrant by ≥4 h from statin |
Enterohepatic Recycling: Effect on Drug Half-Life
Enterohepatic recycling (also termed enterohepatic recirculation) is the pharmacokinetic process by which drugs excreted in bile are reabsorbed from the intestine and returned to the systemic circulation via the portal vein. This cycling effectively extends the apparent half-life of affected drugs because each recirculation event re-introduces drug into the bloodstream, delaying final elimination.
The pharmacokinetic signature of enterohepatic recycling is the appearance of secondary plasma concentration peaks — a second rise in drug levels occurring 4-8 hours after the initial peak, as reabsorbed drug re-enters the circulation. This pattern is observed clinically with ethinylestradiol, morphine (via M6G), and some statins. Enterohepatic recycling increases total drug exposure (AUC) and prolongs the duration of pharmacological effect.
Drugs most significantly affected by enterohepatic recycling include leflunomide (teriflunomide active metabolite — detectable for up to 2 years without cholestyramine washout), ethinylestradiol (oral contraceptive component — bacterial beta-glucuronidase regenerates active steroid), digoxin (minor biliary component), and ezetimibe (cholesterol absorption inhibitor with significant biliary recycling — see PubChem: ezetimibe). The NCBI StatPearls pharmacokinetics chapter covers the mechanistic basis of enterohepatic recycling in detail.