An adverse drug reaction, usually abbreviated ADR, is a harmful and unintended response associated with the use of a medicine. The term is used in clinical practice and medicine regulation to describe unwanted effects that are considered harmful in relation to medicinal treatment. The MHRA describes an adverse reaction as an effect that causes harm and notes that ADRs can occur with prescription medicines, pharmacy medicines, retail medicines, vaccines, herbal products and homeopathic remedies. An ADR is therefore not restricted to a particular class of medicine or to prescription treatment.
An ADR is related to, but not identical to, the everyday phrase side effect. Side effect is a broad term for an additional effect of a medicine, whereas adverse drug reaction emphasises an unintended and harmful response. Some side effects may be unpleasant without meeting a serious clinical threshold, while an ADR concerns a harmful response associated with the medicine. In practice the terminology can overlap, and the MHRA's public information sometimes describes suspected ADRs as side effects. The important distinction is that an ADR represents a safety concern rather than merely an expected pharmacological effect.
An ADR does not necessarily mean that the medicine was definitely responsible for the reaction. Pharmacovigilance systems frequently use the concept of a suspected ADR because a temporal association or clinical suspicion may be enough to warrant reporting and investigation. The MHRA states that people do not have to prove causality before reporting a suspected ADR through the Yellow Card Scheme. Other possible causes, concurrent illnesses, other medicines and interactions may ultimately explain the symptom. Establishing causality is therefore a scientific and clinical assessment rather than something that can always be determined from a single patient's experience.
ADRs can arise through several mechanisms. Some are predictable extensions of the medicine's known pharmacological action and may be related to dose or exposure. Others are less predictable and can involve immune mechanisms, genetic susceptibility, unusual metabolism or other individual biological characteristics. Interactions can also produce ADRs by changing drug concentrations or pharmacological effects. The MHRA specifically notes that interactions, changes in dosage, initiation of treatment and discontinuation after long-term use can all be circumstances in which adverse effects may occur.
ADRs range widely in severity and clinical presentation. They can involve the skin, gastrointestinal tract, nervous system, cardiovascular system, respiratory system, blood, liver, kidneys or immune system. Severe reactions can include anaphylaxis, serious skin reactions, major bleeding, organ injury, severe respiratory reactions or other life-threatening conditions. The MHRA's reporting framework considers a reaction serious when it involves outcomes such as death, life-threatening illness, hospitalisation, congenital abnormality, persistent or significant disability or another medically important event. Seriousness in pharmacovigilance is therefore a defined reporting concept rather than simply a synonym for "very unpleasant".
Recognising and reporting ADRs is a central part of pharmacovigilance, which monitors medicine safety after products enter widespread use. Clinical trials cannot reveal every possible adverse reaction because uncommon effects may require exposure of very large populations or long periods before a pattern becomes visible. The UK's Yellow Card Scheme collects reports from healthcare professionals and members of the public and allows regulators to identify possible new safety signals. When a safety concern is supported by accumulated evidence, regulators may add warnings to product information, change advice about use or take other measures to reduce risk.
ADRs can occur with medicines that have been used for many years and are not necessarily confined to newly introduced products. The MHRA notes that rare or delayed effects may be identified even after a medicine has been available for a long period, and its Yellow Card system continues to monitor established medicines as well as products under additional monitoring. Medicines displaying the inverted black triangle symbol are subject to additional monitoring, and the MHRA encourages reporting of all suspected ADRs associated with these products. The symbol does not mean that the medicine is unsafe; it indicates that additional safety information is being collected.
From a practical perspective, a suspected ADR should be considered in relation to the medicine taken, dose, timing, other medicines and supplements, underlying illnesses and the nature and severity of the reaction. A person experiencing a serious or potentially life-threatening reaction requires appropriate urgent medical care rather than relying solely on a reporting system. For non-emergency concerns, a doctor or pharmacist can assess whether the medicine could be responsible and whether treatment should be changed. Suspected reactions can also be reported through the UK's Yellow Card Scheme, which accepts reports from members of the public as well as healthcare professionals.
