Medication Errors in Ontario: Common Patterns and How Claims Are Proven

Medication errors are among the most common preventable adverse events in healthcare. The harm they cause ranges from inconsequential to fatal. The legal and medical frameworks for understanding them are well-developed, and where a medication error has caused real harm to a patient, the path to a malpractice claim follows the same structure as any other claim: standard of care, causation, and damages, each proven through expert evidence.

This post is a practical guide for patients and families who may have experienced harm from a medication error, and for the healthcare professionals and lawyers who work in this area. It explains the common patterns of medication errors, the points in the medication process where they tend to occur, the high-alert medications that account for a disproportionate share of serious harm, and what it takes to prove a malpractice claim arising from a medication error.

The scale of the problem

Statistics Canada has reported that around two-thirds of Canadians take or are prescribed medication in any given year. The Canadian Pharmacists Association has estimated that more than 600 million prescription medications are dispensed each year in Canada.

Most of those prescriptions are filled, administered, and taken without incident. But at the volumes involved, even small error rates produce a substantial number of medication-related adverse events. Patient safety researchers have repeatedly identified medication errors as one of the most common categories of preventable harm in modern healthcare. Several entries on Canada’s national list of “Never Events” (events that should never occur in the delivery of healthcare) relate specifically to medication errors involving high-alert drugs.

Common types of medication errors

Medication errors fall into recurring patterns. The most common types are:

• Prescribing errors. The wrong drug is prescribed for the patient’s condition, sometimes because the prescriber selected the wrong option from a drop-down menu in an electronic medical record, sometimes because of confusion between similarly named drugs (the so-called “look-alike, sound-alike” problem).
• Dosing errors. The right drug is prescribed but the wrong dose is ordered. Decimal-place errors (10 mg instead of 1 mg) and unit errors are particularly dangerous.
• Omission errors. A medication that should have been prescribed or administered is not, leaving the patient without needed treatment. Anticoagulation that should have been started after surgery, antibiotics that should have been prescribed for an infection, and continuation of chronic medications during hospitalization are common contexts.
• Administration errors. The right drug at the right dose is prescribed, but it is given to the wrong patient, by the wrong route (intravenously instead of intramuscularly, for example), at the wrong rate, or at the wrong time.
• Monitoring errors. A medication that requires ongoing monitoring (warfarin, gentamicin, lithium, methotrexate, and many others) is prescribed but the necessary monitoring is not performed, leading to under-treatment or toxicity.
• Drug interaction and allergy errors. A medication is prescribed without checking for interactions with other drugs the patient is taking, or for documented allergies. Many of these errors are caught by pharmacy software, but some still reach the patient.
• Compliance and protocol errors. Standard medication safety protocols (double-check requirements for high-alert drugs, weight-based dosing protocols for paediatric medications) are not followed.

The ten “rights” of medication administration

Healthcare professionals are taught a set of basic checks before administering any medication. The exact wording varies among institutions, but the core is the same:

• The right patient (verify identity by two identifiers)
• The right drug (verify the drug name on the label against the order)
• The right dose (verify the prescribed dose is appropriate)
• The right route (oral, intravenous, intramuscular, etc.)
• The right time (verify the timing is correct)
• The right reason (the indication is appropriate)
• The right documentation (the administration is recorded)
• The right response (the patient’s response is observed)
• The right form (tablet, suspension, injectable, etc.)
• The right action (what to do if any of the rights is missing)

Each of these is a checkpoint where an error can be caught before it reaches the patient. Where medication errors cause harm, one or more of these rights has typically been missed or assumed rather than verified.

High-alert medications

Some medications cause more harm when errors occur than others. Patient safety organizations have identified groups of “high-alert medications” that warrant particular caution and dedicated safety procedures:

• Anticoagulants (warfarin, heparin, direct oral anticoagulants). Errors in dosing, timing, or in the substitution of one anticoagulant for another can cause life-threatening bleeding or clotting.
• Insulin. Dosing errors and confusion between rapid-acting and long-acting forms are common sources of serious harm.
• Opioids. Overdoses, route confusion (oral versus intravenous), and concurrent prescribing of multiple opioids without coordination produce a substantial share of serious medication-related harm.
• Chemotherapy agents. The narrow therapeutic window and serious adverse effects of these drugs make dosing, scheduling, and route errors particularly dangerous.
• Concentrated electrolytes (potassium chloride, hypertonic sodium chloride). Rapid intravenous administration can be fatal.
• Paediatric medications. Most paediatric dosing is calculated by weight. Calculation errors, particularly tenfold errors, are recurring sources of serious harm.

Errors involving these medications are not necessarily more common than other errors, but they are more likely to produce permanent or fatal harm when they occur.

Where medication errors happen in the process

Medication errors can occur at every step in the medication-use process, from the initial prescribing decision to the moment the medication reaches the patient. The major points of risk are:

• Ordering and prescribing. The largest share of errors occurs at this step, where the prescriber writes the order. Published studies have placed this share at close to half of all medication errors.
• Transcribing and documenting. Where orders are transcribed from one system or record to another, transcription errors can introduce dose, route, or drug changes.
• Dispensing. The pharmacist’s role includes verifying the appropriateness of the order, checking for interactions and allergies, and dispensing the correct drug at the correct dose. Errors at this step can take any of those forms.
• Administering. The healthcare professional administering the drug applies the ten rights. Errors at this step can occur even when the order, dispensing, and documentation are correct.
• Monitoring. After administration, monitoring for response, side effects, and adverse outcomes is the final safety check. Errors at this step typically take the form of failure to act on changes in the patient’s clinical condition or laboratory values.

Nurses and pharmacists intercept a substantial proportion of errors before they reach the patient. Published estimates of the interception rate vary, but somewhere between thirty and seventy percent of ordering errors are caught by these checks. The errors that reach patients are typically those that have made it through the safety net.

Proving a medication error claim

A medication error claim follows the same legal framework as any medical malpractice claim in Ontario. The plaintiff must prove:

1. The healthcare professional fell below the standard a reasonable practitioner would have met in the circumstances (breach of the standard of care)
2. The substandard conduct caused a worse outcome than would otherwise have occurred (causation)
3. The worse outcome produced compensable harm (damages)

Standard of care is often the most straightforward element in medication error cases. The ten rights, the high-alert medications protocols, and the published patient safety guidelines establish well-defined expectations. Where a medication error has occurred, the breach is often documented in the chart itself: the wrong order, the wrong drug, the missed monitoring, the unaddressed interaction.

Causation is more variable. The question is whether the harm the patient suffered was caused by the medication error specifically, rather than by the underlying disease or by other treatment. In some cases the connection is direct, as where a patient suffered an anaphylactic reaction to a medication they were known to be allergic to. In others the connection is more contested. Patients on multiple medications, with multiple chronic conditions, and with adverse outcomes that have multiple potential causes, can present complex causation analyses that turn on detailed expert review.

Damages are often the most difficult element to navigate. Where the error has caused no physical injury, the question becomes whether it has caused a compensable mental injury. The Court of Appeal in Bothwell v London Health Sciences Centre confirmed that a medication error, even a serious and frightening one, does not automatically support a claim where the only harm is psychological upset that does not meet the threshold for mental injury under Saadati v Moorhead. Patients who experience short-term emotional distress after a medication error, without functional impairment or treatment, will generally not have a recoverable claim regardless of how clear the breach was.

The practical implication is that medication error cases are often strong on standard of care and weaker on damages. Cases are stronger where the error caused physical injury, prolonged hospitalization, additional treatment burden, or measurable deterioration in clinical outcome.

Why this matters

For patients and families. A medication error in hospital or in the community is a serious event. Whether it supports a successful malpractice claim depends, more than anything, on whether the error caused additional harm beyond what would otherwise have occurred. The clinical outcome that follows the error needs to be carefully assessed against what would have followed proper administration. That is a treating physician’s or pharmacist’s question as much as a lawyer’s question. The first step is to obtain the medical records, including the medication administration records and pharmacy records, and to seek legal advice from a lawyer experienced in this area.

For healthcare professionals. The recurring patterns of medication errors are well known and largely preventable. The ten rights, the high-alert medications framework, and the structured medication-use process are all designed to catch errors before they reach patients. Where errors occur, the evidence of the breach is often in the chart and in the institutional record. Prompt acknowledgement to the patient, accurate documentation, and a focus on system improvements are the components of a constructive response to an error.

For lawyers screening these cases. Medication error cases are often strong on breach and weaker on damages, particularly where the error was caught and corrected before causing physical harm or where the error caused only short-term psychological distress. The Bothwell line of authority means that a clear breach with only psychological consequences is not enough. The screening question is what compensable harm the error caused, not how clear the breach was.

For more on the legal process for medical malpractice claims in Ontario, see Suing for Medical Malpractice in Ontario: What You Need to Know. For the broader threshold question of whether a particular fact pattern supports a viable claim, see Can I Sue for Medical Malpractice in Ontario? For information on obtaining the records that are essential to any medication error investigation, see How to Get Your Medical Records in Ontario. For the Medication Errors practice page, see the linked overview.