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不良反应（英文：adverse effect），在医学领域，又称为不良作用、不良影响、不良后果，是指一种有害的，人们所不希望出现的，由于某种药物或其他诸如化疗或手术之类的医療所造成的反应、效应、作用、效果、结果或影响。
- Patient Safety Network （页面存档备份，存于互联网档案馆） - includes a glossary and articles on adverse effects, drug reactions, medical error, iatrogenesis, among others.
- Australian Adverse Drug Reactions Bulletin - published bimonthly
- MedEffect Canada (Health Canada)
- Side Effects of Prescription Drugs
- Medication Errors（页面存档备份，存于互联网档案馆）—from the Food and Drug Administration (United States).
- Medical Product Safety Information （页面存档备份，存于互联网档案馆） - MedWatch lists safety alerts for drugs, biologics, devices and dietary supplements, recalls, market withdrawals, public health advisories and links
- Medical Devices Safety （页面存档备份，存于互联网档案馆） National Library of Medicine (Medline Plus, useful lists of conventional drug and medical device articles and websites)
- When Medicine Hurts Instead of Helps - June 1998 report by the Alliance for Aging Research.
- Search AERS Data and Report Drug Side Effects at RxISK.org （页面存档备份，存于互联网档案馆）