[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Pharmacology](https://durgical.com/en/pharmacology/) › Adverse drug reactions # Adverse drug reactions Harm caused by a medicine used normally: why it happens, the main types, and how a suspected reaction is judged and reported. Updated Nov 2, 2026 An adverse drug reaction is an unwanted, harmful effect of a medicine given for a normal purpose. Some reactions are an exaggeration of the drug's own action and are predictable; others depend on the immune system or on rare traits of the patient. Recognising the type guides what to do next: lower the dose, change the drug, or avoid the whole class for life. ## Key points - Type A (augmented) reactions follow from the drug's known pharmacology, depend on dose and are the most common, such as bleeding with an anticoagulant. - Type B (bizarre) reactions are unrelated to the usual action, are not dose-related in the ordinary sense, and include allergy and idiosyncratic toxicity. - Immediate allergic reactions are IgE-mediated (hives, wheeze, anaphylaxis); delayed reactions can be severe skin and organ syndromes such as SJS/TEN and DRESS. - Some severe reactions are tied to genes: carriers of HLA-B*5701 have a high risk of abacavir hypersensitivity, so screening comes first. - Every suspected reaction should be reported to pharmacovigilance; in Colombia that is INVIMA's network. ## Predictable versus unpredictable Type A reactions are the drug doing too much of what it is meant to do, or acting on a second target: sedation with a benzodiazepine, hypoglycaemia with insulin, bleeding with warfarin. They usually improve when the dose is lowered. Type B reactions surprise the prescriber: they appear in a minority, at ordinary doses, and are often more dangerous. Other labels are used for chronic effects, delayed effects (such as cancer or birth defects) and effects on stopping a drug (withdrawal). ## Drug allergy A drug allergy is an immune response. Immediate reactions start within minutes to hours and range from hives to anaphylaxis. Delayed reactions appear days later and can damage skin, blood cells or organs. - A history of penicillin allergy is common, yet on proper evaluation most people who carry the label are not truly allergic. - The label is not harmless: it pushes prescribers towards broader second-line antibiotics, with more resistance and side effects. - After a severe delayed reaction (SJS/TEN, DRESS) the drug is avoided and not re-tested. ## Genes and idiosyncrasy Some reactions only happen in people with a particular gene variant. Abacavir can cause a multi-organ hypersensitivity reaction that is much more likely in carriers of HLA-B*5701; the label asks for screening before the first dose and forbids restarting after a reaction. Drug labels list other gene–drug pairs, such as HLA-B with carbamazepine and allopurinol, and CYP2D6 with codeine (see pharmacogenetics). ## Judging causality Deciding whether the drug really caused the problem uses the same questions every time: did the reaction start after the drug, is it a known effect, did it improve when the drug was stopped (dechallenge), did it return when the drug was given again (rechallenge, rarely done on purpose), and is there another explanation? Structured scales turn these answers into categories such as certain, probable or possible. ## Reporting Trials are too small and too short to show rare or late reactions, so safety knowledge after approval depends on reports from practice. WHO defines pharmacovigilance as the science and activities for detecting, assessing, understanding and preventing adverse effects. In Colombia, health professionals report through INVIMA's Red Nacional de Farmacovigilancia and its electronic forms; early reports are the most useful. ## Memory aid A is for Augmented (too much of the known effect), B is for Bizarre (the patient, not the dose, is unusual). ## Test yourself Think of the answer, then tap the question to see it. ## Bleeding in a patient on warfarin is which type of reaction, and why? Type A: it is an exaggeration of the drug's known anticoagulant action and depends on dose and drug level. ## Why can a false penicillin allergy label harm a patient? It leads to broader second-line antibiotics, which are linked to poorer treatment, more resistance, more side effects and higher cost. ## What test is done before starting abacavir, and why? Screening for HLA-B*5701, because carriers have a much higher risk of a serious, sometimes fatal hypersensitivity reaction. ## What do dechallenge and rechallenge mean in causality assessment? Dechallenge is seeing whether the reaction improves when the drug is stopped; rechallenge is seeing whether it returns when the drug is given again. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/pharmacology/topics/adverse-drug-reactions/) [More Pharmacology questions →](https://durgical.com/en/practice/?f=pharmacology) ## Example drugs - [Abacavir](https://durgical.com/en/pharmacology/abacavir/) - [Amoxicillin](https://durgical.com/en/pharmacology/amoxicillin/) - [Benzylpenicillin (penicillin G)](https://durgical.com/en/pharmacology/benzylpenicillin/) - [Warfarin](https://durgical.com/en/pharmacology/warfarin/) - [Allopurinol](https://durgical.com/en/pharmacology/allopurinol/) - [Codeine](https://durgical.com/en/pharmacology/codeine/) ## Connections ## Related classes 3 - [Beta-lactams](https://durgical.com/en/pharmacology/classes/beta-lactams/) - [Nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs)](https://durgical.com/en/pharmacology/classes/nucleoside-nucleotide-reverse-transcriptase-inhibitors/) - [Vitamin K antagonists](https://durgical.com/en/pharmacology/classes/vitamin-k-antagonists/) [← All of pharmacology](https://durgical.com/en/pharmacology/) Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given. Sources: [Pharmacovigilance](https://www.who.int/teams/regulation-prequalification/regulation-and-safety/pharmacovigilance) (World Health Organization, 2026); [Servicios en Línea Farmacovigilancia - INVIMA](https://www.invima.gov.co/farmacovigilancia) (Instituto Nacional de Vigilancia de Medicamentos y Alimentos (INVIMA), Colombia, 2026); [Clinical Features of Penicillin Allergy](https://www.cdc.gov/antibiotic-use/hcp/clinical-signs/index.html) (Centers for Disease Control and Prevention (CDC), 2026); [DailyMed - ABACAVIR tablet, film coated](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=5409c8ed-17c3-454f-bb88-d597ba8f84ec) (DailyMed (U.S. National Library of Medicine), 2026). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/pharmacology/topics/adverse-drug-reactions/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/pharmacology/topics/adverse-drug-reactions/