What is the most common cause of an adverse drug reaction?

October 13, 2025 •

4 min read

Research indicates that approximately 75-80% of adverse drug reactions (ADRs) are predictable and dose-dependent. The most common cause of an adverse drug reaction is a predictable extension of a drug's known pharmacology, often stemming from issues with dosage or patient-specific factors.

Quick Summary

The most frequent cause of adverse drug reactions is a predictable, dose-related effect of a medication, often resulting from issues like inappropriate dosage or impaired drug clearance. Risk is heightened by polypharmacy, age, and organ dysfunction, along with preventable medication errors.

Key Points

Type A reactions are the most common: The majority of adverse drug reactions (ADRs) are predictable, dose-dependent, and an extension of the drug's known pharmacology.
Polypharmacy is a major risk factor: Taking multiple medications, common in older adults and those with multimorbidity, increases the likelihood of drug interactions and ADRs.
Poor organ function increases risk: Impaired renal or hepatic function can lead to drug accumulation and toxicity, particularly in the elderly.
Proper dosing and monitoring are key: Since Type A reactions are dose-dependent, using the appropriate dose and monitoring drug levels are critical preventative measures.
Medication errors contribute significantly: Preventable errors, such as incorrect prescribing or administration, are a notable cause of adverse drug events.
Patient vigilance is crucial: Patients and caregivers can help prevent ADRs by maintaining accurate medication lists and reporting any unusual symptoms.

An adverse drug reaction (ADR) is an unintended, undesirable, and harmful effect resulting from medication use. These reactions are a significant public health issue, contributing to morbidity and mortality globally. Understanding the root causes is crucial for preventing them and ensuring patient safety. The most common type of adverse drug reaction is classified as Type A, which is predictable, dose-dependent, and related to the drug's primary pharmacological action.

The Dominance of Dose-Related (Type A) Reactions

Accounting for up to 80% of all adverse drug reactions, Type A reactions are a direct consequence of a drug's known mechanism of action. They are considered an exaggerated but expected side effect. The severity often correlates with the dose, meaning higher doses increase the risk and intensity of the reaction. For example, the anticoagulant warfarin is known for its blood-thinning effect, but a Type A reaction would be excessive bleeding resulting from an overly high dose. These reactions are often avoidable with proper dosing and monitoring.

Factors contributing to dose-related reactions

Several factors can influence a patient's risk of a dose-related ADR. These are often related to how the body processes and responds to medication. Key contributors include:

Impaired drug clearance: Conditions affecting the liver or kidneys, such as renal or hepatic impairment, can decrease the body's ability to metabolize or excrete a drug. This leads to higher-than-expected drug concentrations in the bloodstream, increasing the risk of adverse effects. This is a particularly important factor in older adults, whose organ function may naturally decline with age.
Genetic predisposition: An individual's genetic makeup can affect how they metabolize certain drugs. Variations in enzymes, such as the cytochrome P450 system, can result in some people being 'poor metabolizers.' This can lead to drug accumulation and toxicity even at standard doses.
Polypharmacy and drug-drug interactions: Taking multiple medications simultaneously, a condition known as polypharmacy, is a major risk factor for ADRs, particularly in the elderly. The risk of an adverse drug interaction increases exponentially with the number of drugs prescribed. Interactions can affect drug metabolism, excretion, or pharmacological effect, leading to toxic drug levels.

Preventing the most common adverse drug reactions

Given that Type A reactions are largely preventable, several strategies can be employed to mitigate risk. These involve a collaborative effort between healthcare professionals and patients.

Clinical strategies for ADR prevention

Comprehensive medication review: Regular and thorough medication reviews are essential, especially during transitions of care (e.g., hospital admission or discharge). This process, known as medication reconciliation, helps identify and resolve discrepancies, ensuring appropriate dosing and preventing omissions or duplications.
Tailored dosing: Prescribing physicians should consider patient-specific factors like age, weight, and renal and hepatic function when determining medication dosages. The old maxim of "start low and go slow" is particularly relevant for older adults, who are more susceptible to adverse effects.
Consistent monitoring: For medications with a narrow therapeutic index, like warfarin or digoxin, close and consistent monitoring is critical. This helps ensure drug levels remain within the therapeutic window, avoiding both ineffectiveness and toxicity.
Improved communication: Clear communication among healthcare providers and between providers and patients is fundamental. This includes accurately documenting allergies, explaining potential side effects, and ensuring patients understand their medication regimen.

Patient-centered prevention tips

Maintain an updated medication list: Keep a current list of all medications, including over-the-counter drugs, vitamins, and supplements. This list should be shared with every healthcare provider involved in your care.
Use a single pharmacy: Using one pharmacy helps your pharmacist act as a central point for screening potential drug interactions, as they have access to your full medication history.
Take medication as directed: Follow all instructions for timing and dosage precisely. Do not self-adjust doses. If you have questions, ask your pharmacist or doctor.
Be aware of symptoms: Report any unusual symptoms or side effects to your healthcare provider immediately. Subtle changes like confusion or falls in older adults can be signs of an ADR.

Comparison of ADR Types: Type A vs. Type B

While dose-related (Type A) reactions are the most common, it is important to distinguish them from other types of ADRs, particularly the less predictable Type B reactions.


Feature	Type A (Augmented) Reactions	Type B (Bizarre) Reactions
Incidence	Very common (75-80% of all ADRs)	Uncommon (20-25% of all ADRs)
Predictability	Predictable from the known pharmacology	Unpredictable, not related to pharmacology
Dose-dependence	Dose-dependent; more likely and/or severe with higher doses	Not dose-dependent; can occur with very low doses
Mechanism	Exaggeration of normal pharmacological effect	Idiosyncratic or immune-mediated (allergic)
Prevention	Often preventable with appropriate dosing and monitoring	Not preventable, requires prior exposure for allergic reactions
Example	Excessive bleeding with warfarin	Anaphylaxis to penicillin

Conclusion

The most common cause of an adverse drug reaction is a predictable, dose-related effect resulting from the drug's inherent properties. Factors such as a patient's age, comorbidities, reduced organ function, and the number of medications taken (polypharmacy) significantly increase the risk of these Type A reactions. While unpredictable and severe Type B reactions do occur, the majority of ADRs are avoidable with careful attention to dosage, proper patient monitoring, and enhanced communication among healthcare professionals and patients. By focusing on these strategies, the burden of adverse drug reactions can be significantly reduced, leading to safer and more effective medication use.

National Action Plan for Adverse Drug Event Prevention

Frequently Asked Questions

A side effect is a known, often mild and predictable, effect of a drug, which can be beneficial or harmful. An adverse drug reaction (ADR) is a harmful, unintended, and often more severe response to a medication, occurring at doses normally used for treatment.

No, not all ADRs are preventable. Type A (dose-related) reactions are often preventable with proper dosing and monitoring. However, Type B (idiosyncratic or allergic) reactions are unpredictable and generally not preventable, as they are not dependent on the drug's known pharmacology or dose.

Both older adults and children are at heightened risk of ADRs due to age-related physiological differences. In older adults, declining liver and kidney function impairs drug metabolism and excretion, increasing drug concentration and risk of toxicity. In children, dosing is more complex and based on weight.

Polypharmacy, the use of multiple medications, is a major risk factor. It increases the potential for drug-drug interactions, which can alter how drugs are processed by the body and lead to toxic levels or enhanced side effects. The more medications a person takes, the higher the risk.

Symptoms vary widely but can include rash, itching, swelling, nausea, dizziness, confusion, or changes in mental status. In older adults, subtle changes like a new fall or confusion can be a sign of an ADR.

If you suspect an ADR, contact your healthcare provider immediately. If the reaction is severe, such as difficulty breathing or severe swelling, seek emergency medical care. Do not stop taking a prescribed medication without medical advice unless directed to do so by a healthcare professional.

Medication errors can be prevented through clear communication among all healthcare providers, thorough medication reconciliation, and patient education. Patients should also maintain an accurate medication list and follow dosing instructions carefully.