The liver is the body's primary drug-metabolising organ. When hepatic function is impaired — from cirrhosis, hepatitis, or acute liver failure — standard doses of many drugs can become toxic. This guide explains the mechanisms, scoring systems, and clinical dose-adjustment principles for hepatic impairment.
Drug metabolism in the liver occurs in two main phases:
CYP450-mediated oxidation, reduction, and hydrolysis. Key enzymes: CYP3A4 (50% of drugs), CYP2D6, CYP2C9, CYP2C19, CYP1A2, CYP2B6. Adds or exposes a reactive functional group to increase polarity.
Glucuronidation (UGT enzymes), sulfation (SULT), acetylation (NAT), glutathione conjugation, and methylation. Produces polar water-soluble conjugates for renal or biliary excretion.
Liver disease impairs both phases, but the degree varies. Phase I CYP activity falls earlier and more dramatically; phase II glucuronidation (e.g., morphine, lorazepam) tends to be more preserved in early cirrhosis. See the CYP450 clinical guide for enzyme-specific detail.
The Child-Pugh score ↗ is the standard tool used in drug labelling and FDA pharmacokinetic study guidance to classify hepatic impairment severity.
| Parameter | 1 point | 2 points | 3 points |
|---|---|---|---|
| Total bilirubin | <34 µmol/L | 34–50 µmol/L | >50 µmol/L |
| Serum albumin | >35 g/L | 28–35 g/L | <28 g/L |
| INR | <1.7 | 1.71–2.20 | >2.20 |
| Ascites | None | Mild (diuretic-responsive) | Moderate–severe |
| Encephalopathy | None | Grade I–II | Grade III–IV |
| Drug / class | Metabolic pathway | Hepatic risk | Guidance |
|---|---|---|---|
| Statins | CYP3A4 / CYP2C9 | Myopathy; hepatotoxicity | Most contraindicated in active liver disease |
| Warfarin | CYP2C9 / CYP3A4 | Bleeding (reduced clotting factors + CYP) | Use with caution; frequent INR monitoring essential |
| Morphine | UGT2B7 glucuronidation | Sedation; active M6G accumulation | Start low, titrate slowly; M6G can accumulate |
| Diazepam | CYP2C19 / CYP3A4 | Prolonged sedation; encephalopathy | Avoid in cirrhosis; prefer oxazepam (glucuronidation) |
| NSAIDs | CYP2C9 / glucuronidation | Renal vasoconstriction; GI bleeding; HRS | Avoid in cirrhosis (especially ascites) |
| Paracetamol | CYP2E1 / SULT / UGT | NAPQI accumulation; hepatotoxicity | Reduce max dose (≤2 g/day); avoid chronic use in Child-Pugh C |
| Tacrolimus | CYP3A4 / P-gp | Accumulation; nephrotoxicity | Reduce dose in Child-Pugh B/C; TDM mandatory |
| Rifampicin | CYP3A4 inducer | Hepatotoxicity; induces own metabolism | Avoid in significant hepatic impairment |
Drug use in hepatic impairment requires understanding two key pharmacokinetic concepts: hepatic extraction ratio and the distinction between flow-dependent and capacity-limited metabolism. High-extraction drugs (extraction ratio >0.7) — such as propranolol, morphine, lidocaine, and verapamil — are cleared primarily by hepatic blood flow. In cirrhosis, portosystemic shunting dramatically increases their oral bioavailability (reduced first-pass effect), while intrinsic clearance may be relatively preserved. For these drugs, hepatic dosing adjustments must focus on reducing the oral dose to account for increased bioavailability.
Low-extraction drugs (extraction ratio <0.3) — such as diazepam, theophylline, and warfarin — are cleared primarily by intrinsic enzyme capacity. In liver disease, reduced CYP activity directly decreases their clearance, leading to prolonged half-lives and drug accumulation. For these drugs, both dose reduction and interval extension may be required.
Drug metabolism in liver disease is further complicated by reduced albumin synthesis (increasing free fraction of highly bound drugs), altered volume of distribution (ascites increases Vd for hydrophilic drugs), and impaired biliary excretion. The FDA hepatic impairment PK guidance ↗ recommends Child-Pugh classification as the standard for hepatic impairment drug studies, and the EMA hepatic impairment guideline ↗ provides a complementary European perspective. Always consult DailyMed ↗ for drug-specific hepatic dosing recommendations.