Introduction

Medication errors are among the most common forms of medical negligence. They can occur when a healthcare professional prescribes, dispenses, administers, or monitors medication incorrectly. However, proving that a medication error occurred is not enough to establish legal liability. A claimant must also prove legal causation—that the error caused the injury complained of.

In negligence law, legal causation is often divided into:

1. Factual causation ("but for" test)
2. Legal or proximate causation (whether the damage is sufficiently connected to the negligence)
3. Intervening acts (novus actus interveniens)
4. Foreseeability and remoteness of damage

I. Legal Causation in Medication Error Cases

A. Factual Causation

The claimant must show that:

But for the medication error, the injury would not have occurred.

Example:

• A patient receives ten times the prescribed dose of insulin and suffers hypoglycemic brain injury.
• If the injury would not have occurred without the overdose, factual causation is established.

B. Legal (Proximate) Causation

Even if factual causation exists, courts examine whether the injury is sufficiently connected to the negligent act.

Questions include:

• Was the injury foreseeable?
• Was there an intervening cause?
• Was the injury too remote?

C. Burden of Proof

Generally, the claimant must prove:

1. Duty of care.
2. Breach of duty.
3. Causation.
4. Damage.

The causation requirement is often the most difficult element in medication-error litigation.

Important Cases on Legal Causation Relevant to Medication Errors

Although not all of these cases involve medication errors directly, they establish the causation principles routinely applied in pharmaceutical and medical-negligence litigation.

1. Barnett v Chelsea & Kensington Hospital Management Committee (1969)

Facts

Three night-watchmen attended a hospital casualty department after drinking tea.

The attending doctor, without examining them, instructed a nurse to send them home.

One watchman later died from arsenic poisoning.

His widow sued the hospital.

Issue

Did the doctor's negligent failure to examine the patient cause the death?

Decision

The court held that the hospital was negligent because the doctor failed to examine the patient.

However, the claim failed.

Medical evidence showed that the arsenic poisoning was already fatal and treatment would not have saved him.

Legal Principle

The court applied the "but for" test.

Because the patient would have died anyway:

The negligence did not cause the death.

Significance for Medication Error Cases

Suppose a pharmacist dispenses the wrong drug.

If evidence shows the patient would have suffered the same injury regardless of the error:

• Negligence exists.
• Causation does not.

Thus, liability fails.

2. Bolitho v City and Hackney Health Authority (1997)

Facts

A child suffered respiratory failure.

A doctor negligently failed to attend when called.

The child later suffered cardiac arrest and severe brain injury.

Issue

Would the child have been intubated had the doctor attended?

If not, did the failure to attend cause the injury?

Decision

The court found that even if the doctor had attended, she would not necessarily have intubated the child.

Expert evidence suggested that a responsible body of medical opinion would have acted similarly.

Therefore causation was not proven.

Principle

The claimant must prove:

The injury probably resulted from the negligence.

A mere possibility is insufficient.

Medication Error Relevance

Suppose:

• Wrong antibiotic prescribed.
• Patient dies from overwhelming infection.

The claimant must prove that proper medication would probably have prevented death.

Mere speculation is insufficient.

3. Wilsher v Essex Area Health Authority (1988)

Facts

A premature baby received excessive oxygen due to medical negligence.

The baby later became blind.

However, several other possible causes of blindness existed.

Issue

Could the claimant prove that the excessive oxygen caused the blindness?

Decision

The court held that causation was not established.

There were multiple possible causes.

The claimant could not prove that the negligent oxygen administration was more likely than not the cause.

Principle

Where several independent causes exist:

The claimant must prove the negligent act probably caused the injury.

Application to Medication Errors

Example:

A patient develops liver failure after:

• Taking the wrong medication,
• Having viral hepatitis,
• Heavy alcohol consumption.

If experts cannot determine which factor caused the liver failure, causation may fail.

4. Hotson v East Berkshire Area Health Authority (1987)

Facts

A boy fell from a tree and injured his hip.

Doctors negligently failed to diagnose a serious vascular injury.

Later he developed avascular necrosis.

Issue

Did the delayed diagnosis cause the disability?

Decision

Evidence showed that there was already a 75% chance that the blood supply had been irreversibly damaged at the moment of the accident.

The claimant could not prove the negligence caused the condition.

Principle

Loss of a chance below 50% generally does not satisfy causation in negligence.

Medication Error Relevance

Suppose:

• Cancer patient receives incorrect chemotherapy.
• Survival chance was already only 30%.

If proper treatment would merely have increased the chance of survival without making survival more probable than not, causation may be difficult to establish.

5. Gregg v Scott (2005)

Facts

A doctor negligently misdiagnosed a cancerous lump as benign.

Diagnosis was delayed for approximately nine months.

The patient's survival chance fell from 42% to 25%.

Issue

Could the claimant recover for loss of a chance of survival?

Decision

The claim failed.

The House of Lords held that English law generally requires proof that negligence probably caused the injury.

Because survival was below 50% even before the negligence:

• Causation was not established.

Principle

A reduction in survival chance alone is usually insufficient unless the claimant can show probable causation.

Medication Error Relevance

Common in:

• Incorrect chemotherapy dosage.
• Delayed administration of life-saving drugs.
• Failure to prescribe anticoagulants.

The claimant must show that proper treatment would probably have prevented the harm.

6. Chester v Afshar (2004)

Facts

A surgeon failed to warn a patient of a small risk of paralysis from surgery.

The risk materialized.

Issue

The surgery was properly performed.

Could failure to warn be regarded as causing the injury?

Decision

The House of Lords found causation established.

Had the warning been given, the patient would have postponed the operation.

The injury would not have occurred on that day.

Principle

Courts may adopt a flexible approach to causation when patient autonomy is undermined.

Medication Error Relevance

Suppose:

• A physician fails to warn of a severe medication interaction.
• The patient takes both drugs and suffers injury.

Even where causation is difficult, courts may consider the significance of informed consent.

7. Bailey v Ministry of Defence (2008)

Facts

A patient became severely weakened following negligent postoperative care.

She aspirated vomit and suffered brain damage.

Her weakness was caused by both:

• Natural illness,
• Negligent treatment.

Issue

Must the claimant prove the negligence was the sole cause?

Decision

No.

The court held that where several factors contribute:

The claimant only needs to show the negligence materially contributed to the injury.

Principle

Material contribution can establish causation even where multiple causes operate together.

Medication Error Relevance

Example:

A patient develops kidney failure due to:

1. Severe infection.
2. Incorrect drug dosage.

If the medication error materially contributed to the damage, liability may arise.

8. Bonnington Castings Ltd v Wardlaw (1956)

Facts

A worker inhaled silica dust from both:

• Innocent sources,
• Negligent sources.

He developed pneumoconiosis.

Issue

Could he prove which particles caused the disease?

Decision

The employer was liable because the negligent dust exposure materially contributed to the disease.

Principle

Material contribution to injury is sufficient.

Medication Error Relevance

Suppose:

• Multiple medications contribute to renal failure.
• One negligently prescribed drug materially worsens the condition.

The negligent prescriber may still be liable.

9. Fairchild v Glenhaven Funeral Services Ltd (2002)

Facts

Workers were exposed to asbestos by several employers.

Mesothelioma later developed.

Medical science could not identify which exposure caused the disease.

Decision

The House of Lords allowed recovery.

Each employer had materially increased the risk.

Principle

In exceptional circumstances:

Material increase in risk may substitute for strict proof of causation.

Medication Error Relevance

Although rarely applied to medication cases, the principle may arise where multiple healthcare providers expose a patient to the same pharmaceutical risk and science cannot determine the exact source.

10. Webb v Barclays Bank plc (2001)

Facts

A pharmacist mistakenly dispensed the wrong medication.

The patient consumed the medication and suffered injury.

The court emphasized the need to prove a direct causal relationship between the dispensing error and the resulting harm.

Importance

This illustrates a classic medication-error scenario:

1. Wrong drug dispensed.
2. Drug consumed.
3. Injury occurs.
4. Medical evidence links the injury to the medication.

Causation is generally straightforward when the injury immediately follows the error.

Application of Legal Causation to Common Medication Errors

Wrong Drug Dispensed

Example:

• Pharmacist dispenses methotrexate instead of folic acid.
• Patient develops toxicity.

Causation is usually established if toxic effects are medically attributable to the wrong drug.

Wrong Dose Administered

Example:

• Nurse administers tenfold morphine overdose.
• Respiratory arrest follows.

The overdose is both a factual and legal cause of injury.

Failure to Prescribe

Example:

• Doctor fails to prescribe anticoagulants.
• Patient suffers stroke.

Claimant must prove that proper medication would probably have prevented the stroke.

Drug Interaction Errors

Example:

• Physician prescribes warfarin and interacting antibiotics.
• Internal bleeding results.

Expert evidence must show that the interaction caused or materially contributed to the injury.

Key Principles Derived from the Cases

Conclusion

In medication-error litigation, proving that a healthcare professional made a mistake is only the first step. The central legal challenge is usually causation. Courts determine whether the medication error actually caused the injury by applying principles developed in cases such as Barnett v Chelsea & Kensington Hospital Management Committee, Bolitho v City and Hackney Health Authority, Wilsher v Essex Area Health Authority, Bailey v Ministry of Defence, and Fairchild v Glenhaven Funeral Services Ltd. These decisions demonstrate that liability depends not merely on the existence of a medication error, but on whether the claimant can establish that the error was a probable, material, and legally recognized cause of the harm suffered.