Adverse drug reaction

Adverse drug reaction

Adverse drug reaction
A red skin rash due to a drug reaction
Classification and external resources
ICD-10 T88.7, Y40-Y59
ICD-9-CM 995.2, E850-E858
DiseasesDB 295
MeSH D004362

An adverse drug reaction (ADR) is an injury caused by taking a medication.[1] ADRs may occur following a single dose or prolonged administration of a drug or result from the combination of two or more drugs. The meaning of this expression differs from the meaning of "side effect", as this last expression might also imply that the effects can be beneficial.[2] The study of ADRs is the concern of the field known as pharmacovigilance. An adverse drug event (ADE) refers to any injury occurring at the time a drug is used, whether or not it is identified as a cause of the injury.[1] An ADR is a special type of ADE in which a causative relationship can be shown.


  • Classification 1
  • Cause 2
  • Seriousness and severity 3
  • Location 4
  • Mechanisms 5
    • Abnormal pharmacokinetics 5.1
      • Comorbid disease states 5.1.1
      • Genetic factors 5.1.2
        • Phase I reactions
        • Phase II reactions
      • Interactions with other drugs 5.1.3
        • Protein binding
        • Cytochrome P450
    • Synergistic effects 5.2
  • Assessing causality 6
  • Monitoring bodies 7
  • Epidemiology 8
  • See also 9
  • References 10
  • External links 11


ADRs may be classified by e.g. cause and severity.


  • Type A: Augmented pharmacologic effects - dose dependent and predictable
Type A reactions, which constitute approximately 80% of adverse drug reactions, are usually a consequence of the drug’s primary pharmacological effect (e.g. bleeding when using the anticoagulant warfarin) or a low therapeutic index of the drug (e.g. nausea from digoxin), and they are therefore predictable. They are dose-related and usually mild, although they may be serious or even fatal (e.g. intracranial bleeding from warfarin). Such reactions are usually due to inappropriate dosage, especially when drug elimination is impaired. The term ‘side effects’ is often applied to minor type A reactions.[3]

Types A and B were proposed in the 1970s,[4] and the other types were proposed subsequently when the first two proved insufficient to classify ADRs.[5]

Seriousness and severity

The American Food and Drug Administration defines a serious adverse event as one when the patient outcome is one of the following:[6]

  • Death
  • Life-threatening
  • Hospitalization (initial or prolonged)
  • Disability - significant, persistent, or permanent change, impairment, damage or disruption in the patient's body function/structure, physical activities or quality of life.
  • Congenital anomaly
  • Requires intervention to prevent permanent impairment or damage

Severity is a point on an arbitrary scale of intensity of the adverse event in question. The terms "severe" and "serious" when applied to adverse events are technically very different. They are easily confused but can not be used interchangeably, requiring care in usage.

A headache is severe, if it causes intense pain. There are scales like "visual analog scale" that help clinicians assess the severity. On the other hand, a headache is not usually serious (but may be in case of subarachnoid haemorrhage, subdural bleed, even a migraine may temporally fit criteria), unless it also satisfies the criteria for seriousness listed above.


Adverse effects may be local, i.e. limited to a certain location, or systemic, where a medication has caused adverse effects throughout the systemic circulation.

For instance, some ocular antihypertensives cause systemic effects,[7] although they are administered locally as eye drops, since a fraction escapes to the systemic circulation.


Adverse drug reaction leading to hepatitis (drug-induced hepatitis) with granulomata. Other causes were excluded with extensive investigations. Liver biopsy. H&E stain.

As research better explains the biochemistry of drug use, fewer ADRs are Type B and more are Type A. Common mechanisms are:

  • Abnormal pharmacokinetics due to
  • Synergistic effects between either
    • a drug and a disease
    • two drugs

Abnormal pharmacokinetics

Comorbid disease states

Various diseases, especially those that cause renal or hepatic insufficiency, may alter drug metabolism. Resources are available that report changes in a drug's metabolism due to disease states.[8]

Genetic factors

Abnormal drug metabolism may be due to inherited factors of either Phase I oxidation or Phase II conjugation.[9][10] Pharmacogenomics is the study of the inherited basis for abnormal drug reactions.

Phase I reactions

Inheriting abnormal alleles of cytochrome P450 can alter drug metabolism. Tables are available to check for drug interactions due to P450 interactions.[11][12]

Inheriting abnormal butyrylcholinesterase (pseudocholinesterase) may affect metabolism of drugs such as succinylcholine[13]

Phase II reactions

Inheriting abnormal N-acetyltransferase which conjugated some drugs to facilitate excretion may affect the metabolism of drugs such as isoniazid, hydralazine, and procainamide.[12][13]

Inheriting abnormal thiopurine S-methyltransferase may affect the metabolism of the thiopurine drugs mercaptopurine and azathioprine.[12]

Interactions with other drugs

The risk of drug interactions is increased with polypharmacy.

Protein binding

These interactions are usually transient and mild until a new steady state is achieved.[14][15] These are mainly for drugs without much first-pass liver metabolism. The principal plasma proteins for drug binding are:[16]

  1. albumin
  2. α1-acid glycoprotein
  3. lipoproteins

Some drug interactions with warfarin are due to changes in protein binding.[16]

Cytochrome P450

Patients have abnormal metabolism by cytochrome P450 due to either inheriting abnormal alleles or due to drug interactions. Tables are available to check for drug interactions due to P450 interactions.[11]

Synergistic effects

An example of synergism is two drugs that both prolong the QT interval.

Assessing causality

Causality assessment is used to determine the likelihood that a drug caused a suspected ADR. There are a number of different methods used to judge causation, including the

  • ADR online tools
  • Drug Toxicity (Citizendium)
  • Incidence of adverse drug reactions in human immune deficiency virus-positive patients using highly active antiretroviral therapy PMC3312730

External links

  1. ^ a b "Guideline For Good Clinical Practice" (PDF). International Conference on Harmonisation of Technical Requirements for Registration of Pharmaceuticals for Human Use. 10 June 1996. p. 2. Retrieved 12 July 2014. 
  2. ^ a b Nebeker JR, Barach P, Samore MH (2004). "Clarifying adverse drug events: a clinician's guide to terminology, documentation, and reporting". Ann. Intern. Med. 140 (10): 795–801.  
  3. ^ Ritter, J M (2008). A Textbook of Clinical Pharmacology and Therapeutics. Great Britain. p. 62.  
  4. ^ Rawlins MD, Thompson JW. Pathogenesis of adverse drug reactions. In: Davies DM, ed. Textbook of adverse drug reactions. Oxford: Oxford University Press, 1977:10.
  5. ^ Aronson JK. Drug therapy. In: Haslett C, Chilvers ER, Boon NA, Colledge NR, Hunter JAA, eds. Davidson's principles and practice of medicine 19th ed. Edinburgh: Elsevier Science, 2002:147-
  6. ^ "MedWatch - What Is A Serious Adverse Event?". Archived from the original on 29 September 2007. Retrieved 2007-09-18. 
  7. ^ Rang, H. P. (2003). Pharmacology. Edinburgh: Churchill Livingstone.   Page 146
  8. ^ "Clinical Drug Use". Archived from the original on 1 November 2007. Retrieved 2007-09-18. 
  9. ^ Phillips KA, Veenstra DL, Oren E, Lee JK, Sadee W (2001). "Potential role of pharmacogenomics in reducing adverse drug reactions: a systematic review". JAMA 286 (18): 2270–9.  
  10. ^ Goldstein DB (2003). "Pharmacogenetics in the laboratory and the clinic". N. Engl. J. Med. 348 (6): 553–6.  
  11. ^ a b "". Archived from the original on 30 August 2007. Retrieved 2007-09-18. 
  12. ^ a b c Weinshilboum ARE; Collins, Francis S.; Weinshilboum, Richard (2003). "Inheritance and drug response". N. Engl. J. Med. 348 (6): 529–37.  
  13. ^ a b Evans WE, McLeod HL (2003). "Pharmacogenomics--drug disposition, drug targets, and side effects". N. Engl. J. Med. 348 (6): 538–49.  
  14. ^ DeVane CL (2002). "Clinical significance of drug binding, protein binding, and binding displacement drug interactions". Psychopharmacology bulletin. 36 (3): 5–21.  
  15. ^ Benet LZ, Hoener BA (2002). "Changes in plasma protein binding have little clinical relevance". Clin. Pharmacol. Ther. 71 (3): 115–21.  OVID full text summary table at OVID
  16. ^ a b Sands CD, Chan ES, Welty TE (2002). "Revisiting the significance of warfarin protein-binding displacement interactions". The Annals of pharmacotherapy 36 (10): 1642–4.  
  17. ^ Davies EC, Rowe PH, James S, et al. (2011). "An Investigation of Disagreement in Causality Assessment of Adverse Drug Reactions". Pharm Med 25 (1): 17–24.  
  18. ^ Holvey, C; Connolly, A.; Taylor, D. (August 2010). "Psychiatric side effects of non-psychiatric drugs.". British journal of hospital medicine (London, England : 2005) 71 (8): 432–6.  
  19. ^ Otsubo, T (2003). "[Psychiatric complications of medicines].". Ryoikibetsu shokogun shirizu (40): 369–73.  
  20. ^ Weiss AJ, Elixhauser A. Origin of Adverse Drug Events in You.S. Hospitals, 2011. HCUP Statistical Brief #158. Agency for Healthcare Research and Quality, Rockville, MD. July 2013. [3]
  21. ^ a b Weiss A.J., Elixhauser A. Characteristics of Adverse Drug Events Originating During the Hospital Stay, 2011. HCUP Statistical Brief #164. October 2013. Agency for Healthcare Research and Quality, Rockville, MD. [4].


See also

In the You.S., females had a higher rate of ADEs involving opiates and narcotics than males in 2011, while male patients had a higher rate of anticoagulant ADEs. Nearly 8 in 1,000 adults aged 65 years or older experienced one of the four most common ADEs (steroids, antibiotics, opiates and narcotics, and anticoagulants) during hospitalization.[21]

A study by the Agency for Healthcare Research and Quality (AHRQ) found that in 2011, sedatives and hypnotics were a leading source for adverse drug events seen in the hospital setting. Approximately 2.8% of all ADEs present on admission and 4.4% of ADEs that originated during a hospital stay were caused by a sedative or hypnotic drug.[20] A second study by AHRQ found that in 2011, the most common specifically identified causes of adverse drug events that originated during hospital stays in the You.S. were steroids, antibiotics, opiates and narcotics, and anticoagulants. Patients treated in urban teaching hospitals had higher rates of ADEs involving antibiotics and opiates/narcotics compared to those treated in urban nonteaching hospitals. Those treated in private, not-for-profit hospitals had higher rates of most ADE causes compared to patients treated in public or private, for-profit hospitals.[21]


Many countries have official bodies that monitor drug safety and reactions. On an international level, the WHO runs the Uppsala Monitoring Centre, and the European Union runs the European Medicines Agency (EMEA). In the United States, the Food and Drug Administration (FDA) is responsible for monitoring post-marketing studies. In Canada, the Marketed Health Products Directorate of Health Canada is responsible for the surveillance of marketed health products. In Australia, the Therapeutic Goods Administration (TGA) conducts postmarket monitoring of therapeutic products.

Monitoring bodies

Assigning causality to a specific agent often proves difficult, unless the event is found during a clinical study or large databases are used. Both methods have difficulties and can be fraught with error. Even in clinical studies some ADRs may be missed as large numbers of test individuals are required to find that adverse drug reaction. Psychiatric ADRs are often missed as they are grouped together in the questionnaires used to assess the population.[18][19]