Adverse drug reaction (ADR)

Definition

An adverse drug reaction (ADR) is an unintended, undesirable effect produced by a drug at therapeutic doses. ADRs are distinct from effects produced by overdose, drug-drug interactions, or improper use. ADRs range from minor (mild rash, nausea) to severe (liver damage, anaphylaxis, death). Some ADRs are predictable and dose-related (type A reactions), occurring because of the drug's known pharmacological mechanism — for example, hypoglycemia from insulin overdose or gastrointestinal symptoms from GLP-1 agonists (which affect digestive motility). Other ADRs are idiosyncratic and unpredictable (type B reactions), occurring in susceptible individuals due to genetic variations, immune reactions, or unknown mechanisms — for example, severe liver toxicity in some individuals exposed to a drug that is generally well-tolerated. Some ADRs appear only after prolonged exposure or high cumulative doses. Regulatory agencies track ADRs through pharmacovigilance systems in which healthcare providers and patients report ADRs. Accumulation of ADR reports can lead to restrictions on use, dose reductions, or withdrawal of the drug from the market if risks outweigh benefits.

Understanding ADRs is central to risk-benefit assessment of drugs. A drug may be approved despite causing some ADRs because the therapeutic benefit outweighs the risks for the intended population. For example, chemotherapy drugs often cause serious adverse effects because the benefit (cancer treatment) outweighs the risks. However, the same ADRs would be unacceptable for a minor indication where alternative treatments exist. Regulatory agencies assess whether ADRs are proportionate to the therapeutic benefit and whether they can be managed through dose adjustments, monitoring, or patient selection.

For research peptides, characterization of ADRs is important because these compounds are often not extensively studied in large populations. Animal studies may reveal unexpected ADRs not apparent in small early human studies. Post-marketing surveillance (after a drug is approved and marketed) continues to identify ADRs that occur infrequently or that emerge only after longer exposure than was studied in trials. Healthcare providers and patients are encouraged to report suspected ADRs to regulatory agencies, contributing to population-level safety monitoring.