Drug-induced liver injury and paracetamol toxicity

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Predictable intrinsic toxicity, exemplified by paracetamol, relates to dose and toxic metabolite formation. Idiosyncratic DILI is less predictable and may be hepatocellular, cholestatic, mixed or autoimmune-like; antibiotics, antiepileptics, immune therapies and supplements are common categories but any agent needs temporal scrutiny.

The central paracetamol decision is whether acetylcysteine is indicated under the current National Poisons Information Service guidance. Exact nomogram, weight ceiling, infusion and stopping rules are live protocols because errors in timing or calculation cause avoidable harm.

The central DILI decision is whether the suspected agent should be stopped immediately and whether severity warrants admission or specialist referral. Jaundice with hepatocellular injury, rising INR, systemic hypersensitivity or encephalopathy is more concerning than an isolated mild enzyme change.

Key points

Drug-induced liver injury is a diagnosis of careful exclusion based on exposure, latency, biochemical pattern, dechallenge and competing causes; there is no single confirmatory blood test.
Record prescription, non-prescription, herbal, bodybuilding, weight-loss, recreational and recently stopped products, with start dates, dose changes and last doses.
Use the R ratio at first recognition to describe hepatocellular, cholestatic or mixed injury, but interpret laboratory upper limits and clinical context rather than relying on the label alone.
Stop the suspected non-essential agent promptly in significant injury and avoid deliberate rechallenge when the prior reaction was serious.
A single acute paracetamol ingestion with reliable timing can use the UK treatment nomogram from four hours; staggered, repeated or uncertain ingestions must not be plotted as one event.
If more than eight hours have elapsed after a potentially toxic ingestion, start acetylcysteine while obtaining the concentration and advice because delay reduces protection.
A low or undetectable late paracetamol concentration does not exclude established toxicity, and the nomogram does not determine treatment in a symptomatic late presenter.
Report suspected serious or unusual adverse drug reactions through the MHRA Yellow Card scheme and give the patient a precise written avoidance plan.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Predictable paracetamol toxicity

Excess acute or staggered paracetamol exposure overwhelms safe metabolism; therapeutic misadventure can occur through combined products, malnutrition or delayed recognition.

Idiosyncratic medicine injury

Many prescribed medicines cause unpredictable hepatocellular, cholestatic or mixed injury through host-specific metabolic or immune susceptibility rather than simple dose dependence.

Herbal and recreational exposures

Supplements, bodybuilding or weight-loss products and recreational drugs may be omitted unless asked about directly and can contain uncertain or multiple hepatotoxins.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Reactive metabolite accumulation
In paracetamol toxicity, glutathione depletion permits a reactive metabolite to bind hepatocyte proteins and cause centrilobular necrosis.
2
Idiosyncratic cellular injury
Other agents cause mitochondrial, bile-transport or immune-mediated damage after variable latency, producing diverse biochemical and histological patterns.
3
Loss of hepatic function
Extensive necrosis impairs coagulation, glucose control and toxin clearance, leading to encephalopathy, acidosis and acute liver failure.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Hepatocellular DILIRed flag

ALT predominates relative to alkaline phosphatase and symptoms may include malaise, nausea, right upper-quadrant discomfort and jaundice. Severe cases can progress rapidly to coagulopathy and encephalopathy.

Cholestatic or mixed DILI

Pruritus and jaundice with disproportionate alkaline phosphatase rise may persist for weeks after withdrawal. Ultrasound is still needed to exclude mechanical obstruction and infiltrative disease.

Immune-allergic phenotypeRed flag

Fever, rash, facial oedema, eosinophilia, lymphadenopathy or other organ involvement suggests a hypersensitivity syndrome such as DRESS and needs urgent multispecialty assessment.

Early paracetamol poisoningRed flag

Nausea, vomiting or no symptoms can occur in the first day, so clinical wellness cannot replace timed concentration and toxicology assessment. Right upper-quadrant pain and rising ALT develop later.

Established paracetamol hepatotoxicityRed flag

Marked aminotransferases, INR prolongation, metabolic acidosis, lactataemia, hypoglycaemia, kidney injury and encephalopathy indicate severe toxicity even if the drug concentration is now low.

Staggered or repeated supratherapeutic exposureRed flag

Multiple doses over more than an hour, therapeutic duplication or chronic excess creates unreliable timing and cannot be assessed with the single-ingestion nomogram; follow TOXBASE laboratory and treatment criteria.

Competing diagnosis

Viral, autoimmune, ischaemic, biliary, vascular and metabolic disease can mimic DILI. Hypotension, gallstones or hepatitis exposure should not be ignored because a new medicine is present.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Timed plasma paracetamol concentrationFirst step
Why
Guide treatment after a single acute ingestion with reliable timing.
Interpretation and limitations
Draw at least four hours after ingestion and plot only when the event is genuinely single and timed. Use the current UK nomogram and TOXBASE; earlier samples, staggered doses and uncertain times require a different pathway.
02
ALT, AST, ALP, bilirubin and R ratio
Why
Characterise liver-injury pattern at first presentation.
Interpretation and limitations
Calculate R as ALT divided by its upper limit over ALP divided by its upper limit: hepatocellular is at least five, cholestatic at most two, and mixed lies between. Pattern guides exclusion tests but not severity alone.
03
INR, glucose, blood gas, lactate and renal profile
Why
Detect impaired synthesis and multiorgan toxicity.
Interpretation and limitations
Rising INR, acidosis, persistent lactate, hypoglycaemia or creatinine despite treatment prompts critical-care and transplant-centre escalation. Check potassium and bicarbonate during acetylcysteine and renal support.
04
Complete medication and product timeline
Why
Establish latency, dose, dechallenge and hidden repeated exposure.
Interpretation and limitations
Include dispensing records, combination analgesics, online supplements and stopped medicines. Compare known injury signatures cautiously; temporal plausibility strengthens but does not prove causality.
05
Viral, autoimmune and metabolic liver screen
Why
Exclude common and treatable mimics of DILI.
Interpretation and limitations
Select hepatitis serology, autoantibodies, IgG, iron and Wilson tests by context. Acute severe autoimmune hepatitis can have weak serology and still needs hepatology consideration.
06
Ultrasound liver with Doppler
Why
Exclude obstruction, vascular disease and established chronic liver morphology.
Interpretation and limitations
Biliary dilatation redirects the pathway toward duct imaging or intervention. Normal ultrasound does not confirm DILI and does not reduce concern about worsening synthetic function.
07
Serial paracetamol and liver blood tests
Why
Determine whether antidote can stop or must continue.
Interpretation and limitations
At the protocol-defined review point, use paracetamol detectability, ALT trajectory, INR, symptoms and other criteria from TOXBASE or the local pathway; do not stop solely because the scheduled bags have finished.
08
Liver biopsy in selected uncertain cases
Why
Clarify autoimmune-like, infiltrative or persistent cholestatic injury when management would change.
Interpretation and limitations
Histology rarely identifies a unique culprit. Choose a safe specialist route in coagulopathy and never delay antidote or transplant referral for biopsy.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Viral hepatitis

Exposure history, serology and nucleic-acid testing identify viral disease; temporal association with a medicine alone is insufficient to diagnose DILI.

Ischaemic liver injury

Shock, hypoxaemia or cardiac failure with a rapid enzyme rise and fall favours hypoxic hepatitis over an idiosyncratic drug reaction.

Autoimmune or obstructive disease

Immunoglobulins, autoantibodies and bile-duct imaging help distinguish autoimmune hepatitis and biliary obstruction, both of which can coexist with medicine exposure.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01OverdoseSingle acute timed paracetamol ingestionFirst stepOne ingestion event with a reliable time and presentation before established hepatotoxicity.

1Establish exact time, amount, formulation, co-ingestants and body weight; assess self-harm risk and obtain a plasma concentration at or after four hours with baseline liver, renal and coagulation tests.
2If presentation is beyond eight hours after a potentially toxic dose, start acetylcysteine immediately while results are pending; otherwise use the current UK nomogram and TOXBASE treatment line.
3Prescribe the locally authorised acetylcysteine regimen using checked weight-based tables and independent calculation safeguards, monitoring closely for infusion reactions and fluid volume.
4At the defined endpoint, repeat required tests and continue acetylcysteine if the current stopping criteria are not met; seek NPIS or hepatology advice for any ambiguity.

02UncertainStaggered, repeated or late exposureDoses spread over time, unreliable history, late symptoms or abnormal liver tests.

1Do not plot the concentration on the single-ingestion nomogram; contact TOXBASE or NPIS and obtain paracetamol, ALT, INR, creatinine, glucose, acid-base and lactate results promptly.
2Start acetylcysteine without waiting when the current toxicology criteria or clinical risk indicate treatment, especially in a symptomatic late presenter or one with liver injury.
3Search for therapeutic duplication, fasting, intercurrent illness, alcohol dependence and deliberate self-harm while recognising that historical risk categories do not replace the current national protocol.
4EscalationEscalate worsening coagulopathy, acidosis, renal injury, hypoglycaemia or altered consciousness to critical care and a transplant centre, repeating therapy until specialist stopping criteria are fulfilled.

03DILISuspected idiosyncratic liver injuryNew liver-test abnormality temporally associated with a medicine or supplement.

1Assess symptoms and severity, calculate the biochemical pattern, stop the likely non-essential culprit and all unnecessary hepatotoxins, and admit patients with jaundice plus hepatocellular injury or impaired synthesis.
2Build a dated exposure table and exclude viral, autoimmune, biliary, vascular, ischaemic and metabolic alternatives with targeted tests and imaging.
3Discuss severe, prolonged or diagnostically uncertain injury with hepatology; use corticosteroids only for a defined specialist indication such as convincing autoimmune-like disease, not routine DILI.
4Trend to resolution, report through MHRA Yellow Card, document the generic and brand product clearly and communicate avoidance and safe alternatives to primary care and pharmacy.

04ReactionAcetylcysteine infusion reactionFlushing, urticaria, wheeze or hypotension during intravenous antidote.

1Stop or pause the infusion according to severity while assessing airway, breathing and circulation; treat anaphylaxis through the current emergency protocol when diagnostic criteria are met.
2Recognise that many reactions are non-IgE anaphylactoid events and that acetylcysteine remains essential when poisoning risk persists; obtain senior toxicology advice promptly.
3Restart at the protocol-advised rate once symptoms resolve when safe, documenting the event and avoiding a permanent allergy label that could block future antidote.
4Complete the required course and laboratory review, because an infusion reaction does not neutralise the underlying paracetamol toxicity.

Key medicines and prescribing safety3 treatments · regimens, roles and cautions

Prevents or limits paracetamol hepatotoxicity and may be continued in established injury.

Intravenous acetylcysteine

Use the current TOXBASE-approved weight table and locally adopted infusion regimen.

Calculation, weight ceiling and fluid errors can harm. Monitor for anaphylactoid reactions, do not abandon essential treatment after a manageable reaction, and extend therapy when stopping criteria are unmet.

Reduces absorption after selected recent substantial ingestions, including modified-release or delayed-gastric-emptying situations.

Activated charcoal

Give only within the time window and dose advised by TOXBASE.

Protect the airway and avoid use with reduced consciousness unless secured. It does not replace acetylcysteine and may be inappropriate when aspiration or gastrointestinal obstruction risk is high.

Treats airway, breathing or circulation compromise during a severe acetylcysteine-associated reaction.

Intramuscular adrenaline for anaphylaxis

Use the current Resuscitation Council UK anaphylaxis algorithm and repeat if required.

Simple flushing or nausea alone may not be anaphylaxis. Continue physiological monitoring and obtain toxicology advice about safe acetylcysteine resumption after stabilisation.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Acute liver failure

Severe hepatocyte loss causes coagulopathy, hypoglycaemia, encephalopathy, renal injury and a narrow window for transplant-centre assessment.

Prolonged cholestasis or chronic injury

Some idiosyncratic reactions resolve slowly or leave duct loss and fibrosis, requiring monitoring after the suspected agent is stopped.

Severe recurrence on rechallenge

Re-exposure can trigger faster, more intense injury, so deliberate rechallenge after a serious reaction is generally unsafe and prevention of accidental reuse is important.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

During acetylcysteine, observe airway, respiratory status, blood pressure, rash, vomiting, infusion line and prescribed fluid volume, with rapid access to anaphylaxis treatment.
Repeat paracetamol concentration, ALT, INR, creatinine and other protocol tests at the specified decision point, and document explicitly why treatment stops or continues.
For severe toxicity, trend glucose, lactate, pH, electrolytes, urine output and mental state frequently and share deterioration with the transplant centre in real time.
For idiosyncratic DILI, follow bilirubin, ALT, ALP and INR until convincing improvement, then to resolution or a stable chronic cholestatic outcome under specialist review.
Before discharge after deliberate overdose, complete psychosocial assessment, safeguarding and a safe medicines plan; after accidental duplication, reconcile every paracetamol-containing product.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Four hours has meaning

A concentration drawn too early may precede full absorption and cannot be plotted reliably. The exception is not to wait when delayed presentation already triggers empirical antidote.

Nomograms have borders

The UK line applies to a single acute timed ingestion in the defined interval. Staggered exposure, unknown time and established injury demand TOXBASE criteria instead.

Late levels can be low

Paracetamol may be cleared from plasma after initiating hepatic injury. Severe biochemical toxicity with a low level still needs acetylcysteine and transplant-risk assessment.

R ratio describes pattern

Hepatocellular, cholestatic and mixed categories organise differentials and expected course; they do not identify the culprit or substitute for INR and clinical severity.

Dechallenge is not proof

Improvement after stopping a medicine supports causality but coincidental viral or ischaemic injury also improves. A structured exclusion and timeline remain necessary.

Rechallenge can be catastrophic

Re-exposure may produce faster and more severe injury. Avoid it after serious suspected DILI unless an expert judges a unique benefit and monitoring plan outweighs risk.

11Common pitfallsFrequent interpretation and management errors.

01
Plotting a staggered or uncertain ingestion on the acute nomogram.
02
Waiting for a paracetamol result when presentation beyond eight hours already warrants treatment.
03
Stopping acetylcysteine automatically when the scheduled infusion ends.
04
Diagnosing DILI without excluding viral, autoimmune, biliary and ischaemic disease.
05
Recording a vague allergy instead of naming the culprit product and injury.
06
Rechallenging a patient after severe hepatocellular DILI without specialist justification.