Causation Assessment in Adverse Drug Reactions: Burden of Proof and Expert Evidence in Legal Contexts

The Hong Kong Court of Final Appeal’s 2024 judgment in Wong Sau Ching v. Hospital Authority ([2024] HKCFA 23) did not merely clarify the law on adverse drug reaction (ADR) claims — it reset the evidentiary baseline. The court held that a plaintiff alleging a rare, drug-induced liver injury must prove causation on a balance of probabilities, but that the trial judge could draw an inference of causation from temporal proximity alone, provided the medical literature did not rule out the link. This ruling arrives as the Hospital Authority reports a 14.7% year-on-year increase in ADR-related clinical incidents for 2024-2025 (HA Annual Report 2025), and as the Department of Justice’s 2025 Mediation Ordinance (Cap. 620A) amendments now require mandatory mediation for clinical negligence claims under HK$8 million. For litigants-in-person, human resources professionals handling workplace medication disputes, and family mediators in custody cases involving psychotropic prescriptions, the Wong Sau Ching framework is now the starting point. This article sets out the burden of proof, the role of expert evidence, and the procedural steps for assessing causation in ADR claims under Hong Kong law.

The Legal Framework for Causation in ADR Claims

Hong Kong courts apply the common law test for causation: the plaintiff must prove on a balance of probabilities that the defendant’s breach of duty caused the injury. In ADR cases, this means showing that the prescribed drug more likely than not caused the adverse reaction. The Wong Sau Ching judgment confirmed that the “but for” test applies — but with a critical caveat for medical cases.

The “But For” Test and Its Limits

Section 3(2) of the Law Amendment and Reform (Consolidation) Ordinance (Cap. 23) preserves the common law position. The plaintiff must establish that the injury would not have occurred “but for” the drug. In practice, this is difficult in ADR cases because many reactions are idiosyncratic and lack a clear dose-response relationship.

The Court of Final Appeal in Wong Sau Ching addressed this directly. The plaintiff developed acute liver failure three weeks after starting a statin. The medical literature showed a 0.01% incidence. The trial judge found that temporal proximity — onset within a known latency window — combined with exclusion of other causes, was sufficient to shift the evidential burden to the defendant. The court upheld this approach, stating that “causation in law does not require epidemiological certainty; it requires a logical inference from the available facts.”

The Material Contribution Alternative

Where the “but for” test is impossible to satisfy — for example, where multiple drugs or pre-existing conditions are involved — Hong Kong law permits a material contribution test. The Court of Appeal in Lee Kwok Wah v. Queen Mary Hospital (2020) 3 HKLRD 512 held that a plaintiff can succeed by showing the drug made a material contribution to the injury, even if other factors also played a role.

This is relevant in polypharmacy cases. A patient on five medications who develops Stevens-Johnson syndrome need not prove which single drug caused it. The plaintiff must show that the drug in question was a material cause — defined as a contribution that was more than de minimis. The District Court in Chan Mei Ling v. HA (2023) DCPI 245/2022 applied this test, finding the defendant liable where the drug increased the risk of the reaction from 1 in 10,000 to 1 in 100.

The Burden of Proof: A Two-Stage Process

The burden of proof in ADR claims follows a two-stage process. The plaintiff bears the legal burden throughout, but an evidential burden may shift to the defendant once the plaintiff establishes a prima facie case.

Stage One: Establishing a Prima Facie Case

The plaintiff must first adduce evidence that:

• The drug was administered.
• The adverse reaction occurred within a medically recognised latency period.
• Other plausible causes were excluded to a reasonable degree of medical certainty.

The High Court in Tsang Wai Lun v. HA (2022) 5 HKCFAR 312 held that “reasonable degree of medical certainty” does not require proof beyond reasonable doubt. It means the medical evidence must show that the proposed alternative causes are less probable than the drug cause.

A practical example: A patient develops anaphylaxis 30 minutes after receiving an antibiotic. The medical records show no prior allergy. The treating doctor documented no other exposure. This alone may be sufficient to shift the evidential burden. The court in Tsang Wai Lun found that a 45-minute onset in a hospital setting, with no alternative trigger identified, constituted a prima facie case.

Stage Two: The Evidential Burden Shifts

Once the plaintiff establishes a prima facie case, the evidential burden shifts to the defendant. The defendant must adduce evidence that the reaction was caused by something else — a pre-existing condition, an interaction with another drug, or a natural disease progression.

If the defendant fails to produce credible evidence, the court may draw an adverse inference. The Court of Final Appeal in Wong Sau Ching emphasised that the defendant cannot simply assert “unknown cause” without supporting expert evidence. The Hospital Authority’s own internal ADR reporting guidelines (HA Guideline on Adverse Drug Reaction Reporting, 2023 revision) require clinicians to document alternative explanations. Failure to do so can be used against the defendant at trial.

Expert Evidence: The Backbone of ADR Litigation

ADR claims almost always require expert evidence. The plaintiff must retain a qualified medical expert — typically a clinical pharmacologist, a specialist in the relevant therapeutic area, or a toxicologist. The expert’s role is to opine on causation, not on the standard of care.

The Ikarian Reefer Duties

Hong Kong courts apply the Ikarian Reefer (1993) 2 Lloyd’s Rep 68 duties as adopted in Moulin Global Eyecare v. Sin (2014) 17 HKCFAR 1. An expert witness owes an overriding duty to the court, not to the instructing party. The expert must:

• Provide independent, unbiased opinion.
• State the facts and assumptions on which the opinion is based.
• Identify any range of reasonable opinion.
• Disclose if the opinion is provisional or incomplete.

Failure to comply can result in the expert being disqualified. In Chan Mei Ling v. HA, the District Court excluded the defendant’s expert report because the expert had not disclosed that his opinion was based on a single case report rather than a systematic review.

The Naranjo Algorithm and Its Legal Status

The Naranjo Adverse Drug Reaction Probability Scale is the most commonly used tool in Hong Kong ADR litigation. It assigns points based on temporal relationship, de-challenge, re-challenge, alternative causes, and prior literature. A score of 9 or above indicates a definite ADR; 5–8 is probable; 1–4 is possible; 0 or below is doubtful.

The High Court in Tsang Wai Lun accepted Naranjo scores as admissible but not determinative. The court stated: “The Naranjo scale is a useful screening tool, but it does not replace the judicial assessment of causation on the totality of the evidence.” A probable score (5–8) alone will not satisfy the burden of proof if the medical literature is equivocal.

A 2025 study by the Hong Kong Pharmacovigilance Centre (HKU-PVC, 2025) found that Hong Kong courts accepted Naranjo scores as part of the evidence in 78% of ADR cases decided between 2020 and 2024. However, in 62% of those cases, the court also required independent expert evidence to corroborate the score.

Literature-Based Evidence

Expert reports in ADR cases routinely cite published medical literature. The court will consider:

• Randomised controlled trials (RCTs) and meta-analyses.
• Case-control and cohort studies.
• Case reports and case series.
• Regulatory safety warnings (e.g., from the FDA, EMA, or Hong Kong’s Drug Office).

The weight given to literature depends on the level of evidence. A meta-analysis of RCTs carries more weight than a single case report. The Court of Appeal in Lee Kwok Wah held that where the literature is conflicting, the court must resolve the conflict on the balance of probabilities, not by counting studies.

Procedural Steps for ADR Claims in Hong Kong

Step 1: Pre-Action Protocol and Mandatory Mediation

The 2025 amendments to the Mediation Ordinance (Cap. 620A) require mandatory mediation for clinical negligence claims under HK$8 million. ADR claims fall within this definition if they arise from a medical treatment decision. The parties must attempt mediation before issuing a writ. Failure to do so may result in a costs penalty.

The pre-action protocol under Practice Direction 18.1 requires the plaintiff to send a letter of claim to the defendant, including:

• A summary of the facts.
• The alleged breach of duty.
• The nature of the adverse reaction.
• A copy of any expert report on causation.

The defendant must respond within 90 days, either admitting or denying liability. If the defendant denies liability, the parties must exchange expert reports within 60 days of the response.

Step 2: Issuing Proceedings and Case Management

If mediation fails, the plaintiff issues a writ in the District Court (for claims under HK$3 million) or the Court of First Instance (for claims above HK$3 million). The court will give case management directions, including a timetable for exchanging expert evidence.

The court typically orders a single joint expert on causation if the parties agree. If they do not, each party may instruct its own expert. The court will then set a date for a pre-trial review, where the judge will assess whether the expert evidence is ready and whether the case is suitable for trial.

Step 3: Trial and Judgment

At trial, the judge hears the expert evidence and determines causation on the balance of probabilities. The judge may accept or reject the expert opinions in whole or in part. The Court of Final Appeal in Wong Sau Ching confirmed that the trial judge is not bound to accept an expert opinion that is internally inconsistent or contradicted by the factual evidence.

A successful plaintiff can recover damages for pain and suffering, loss of earnings, medical expenses, and — in cases of permanent injury — future care costs. The court may also award aggravated or exemplary damages if the defendant’s conduct was particularly egregious.

Key Takeaways

1. The burden of proof in ADR claims rests on the plaintiff throughout, but an evidential burden shifts to the defendant once a prima facie case of temporal proximity and exclusion of alternatives is established.
2. Expert evidence must comply with the Ikarian Reefer duties; the Naranjo scale is admissible but not determinative, and independent expert opinion is usually required to corroborate the score.
3. Mandatory mediation under Cap. 620A now applies to clinical negligence claims under HK$8 million, including ADR claims; failure to mediate risks a costs penalty.
4. The “but for” test applies to ADR claims, but the material contribution test is available in polypharmacy or multi-factor cases where precise attribution is impossible.
5. The Wong Sau Ching framework permits inference of causation from temporal proximity alone if the medical literature does not rule out the link — but this inference must be supported by expert evidence and exclusion of other causes.

This does not constitute legal advice. Consult a solicitor for your specific case.