What happens when a pharmacist dispenses a different medication than the one prescribed, i.e., one medication instead of another?

Medication Dispensing Errors: Clinical Consequences and Prevention

When a pharmacist dispenses a different medication than prescribed, this represents a critical medication error that places patients at significant risk for adverse drug events, therapeutic failure, unintended drug interactions, and potential harm—with an estimated 4-6% of such discrepancies being clinically significant enough to cause patient injury. 1

Immediate Clinical Risks

When the wrong medication is dispensed, patients face multiple serious consequences:

• Adverse drug events from receiving an unintended medication that may cause allergic reactions, toxicity, or organ damage 1
• Therapeutic failure from not receiving the prescribed medication, leading to uncontrolled disease states (e.g., uncontrolled hypertension, untreated infections) 1
• Drug-drug interactions when the wrong medication interacts with the patient's other medications 1
• Over- or under-dosage if a similar-sounding medication with different dosing is dispensed 1

The British Journal of Pharmacology emphasizes that 67% of medication histories contain at least one prescription error, with 22% having potential to cause significant patient harm. 1

High-Risk Scenarios

Certain situations dramatically increase the likelihood and severity of dispensing errors:

• Look-alike/sound-alike medications are the most common cause of wrong-drug dispensing errors 2, 3
• Elderly patients (≥65 years) face compounded risk due to polypharmacy, with age (OR=1.02 per year) and medication count (OR=1.10 per medication) independently predicting medication errors 1
• High-risk medications including anticoagulants, insulin, and opioids pose particular danger when dispensed incorrectly 1, 4
• High-volume pharmacy settings show positive correlation between prescriptions filled per hour and dispensing error risk (r=0.285, p<0.001) 5

Prevention Strategies

The American Heart Association recommends implementing bar-coded medication administration systems, which reduce medication errors by at least 50%. 2

System-Level Interventions

• Pharmacist verification of all prescriptions before dispensing, as pharmacists obtain more accurate medication histories than physicians 1, 2
• Tall Man lettering and visual alerts in pharmacy information systems to prevent look-alike drug confusions 2, 3
• Alert systems for drug name and strength confusion, though these must be carefully implemented to avoid alert fatigue 3
• Standardized dispensing protocols with double-check systems for high-risk medications 1

Patient-Level Safeguards

• Patient education to recognize their medications by appearance and indication, enabling patients to catch errors before taking the wrong medication 2
• Medication reconciliation at every care transition, which is particularly critical for high-risk populations 1

Common Pitfalls to Avoid

Mail service pharmacies (risk score 1.85±1.32), traditional chain pharmacies (1.66±1.18), and hospital pharmacies (1.61±1.09) report significantly higher dispensing error rates than independent pharmacies (0.75±0.84). 5

Critical factors contributing to wrong-medication dispensing include:

• Communication failures between prescribers and pharmacists 6
• Work overload and inadequate staffing ratios 6, 5
• Distraction and interruption during the dispensing process 1, 6
• Poor package labeling that creates confusion 6
• Inadequate pharmacist training in medication safety protocols 1

Immediate Actions When Error Discovered

• Do not administer the incorrect medication if caught before patient receives it 7
• Report the error through non-punitive incident reporting systems, as approximately 20% of medication errors are near-misses caught before reaching patients 7
• Document thoroughly including the witness verification and exact circumstances 7
• Contact the prescriber immediately if patient has already received the wrong medication to determine clinical management 1

The evidence strongly supports that involving clinical pharmacists throughout the medication process—from dispensing to administration—significantly reduces medication errors and improves patient safety outcomes. 1, 8