What is an adverse drug reaction?
- An adverse drug reaction (ADR) is an unwanted or harmful reaction experienced following the administration of a drug or combination of drugs under normal conditions of use, which is suspected to be related to the drug.
- The reaction may be a known side effect of the drug or it may be new and previously unrecognised.
Is the reaction an ADR or an adverse event?
‘Adverse reactions' and 'adverse events' are not always the same.
- An adverse event is any undesirable event experienced by a patient whilst taking a medicine, regardless of whether or not the medicine is suspected to be related to the event. An example of an adverse event is a patient being hit by a car while on a specific medication.
Whereas:
- An adverse drug reaction is any undesirable experience that has happened to the patient while taking a drug that is suspected to be caused by the drug or drugs. An example of an ADR could be a patient experiencing anaphylaxis shortly after taking the drug.
Type A Reactions
Type A (augmented) reactions result from an exaggeration of a drug’s normal pharmacological actions when given at the usual therapeutic dose and are normally dose-dependent.
Examples include:
- Low blood pressure with antihypertensives
- Low blood sugar with insulin
Type A reactions also include those that are not directly related to the desired pharmacological action of the drug (e.g. dry mouth that is associated with tricyclic antidepressants).
Type B Reactions
Type B (bizarre) reactions are novel responses that are not expected from the known pharmacological actions of the drug.
- Anaphylaxis with penicillin
- Skin rashes with antibiotics
Characteristics of Type A and Type B reactions:
|
Type A
|
Type B
|
|---|---|
|
Predictable |
Unpredictable
Rarely dose dependent Low morbidity High mortality Responds to drug withdrawal |
What does the UK Yellow Card Scheme collect – events or reactions?
The Yellow Card Scheme relies upon voluntary reporting of suspected adverse drug reactions. This means that you should therefore only report those cases where you have a suspicion that there is a causal relationship between the medicinal product taken and the suspected reaction experienced. However if in doubt about causality, please go ahead and report the reaction.
How to identify adverse drug reactions
Patients may tell you about symptoms they have experienced since taking a new medicine, and it is important to listen to the patient’s own concerns regarding their drug therapy. However, as some adverse reactions may not be apparent to the patient, you will need to be alert to the possible occurrence of ADRs.
Your own observations and initiative will be vital in this respect, in linking a sign or symptom to either current or previous therapy. (Remember these can include over-the-counter (OTC) drugs and unlicensed herbal remedies).
Other things to be alert for include:
- Abnormal clinical measurements (e.g. temperature, pulse, blood pressure, blood glucose, body weight) while on drug therapy
- Abnormal biochemical or haematological laboratory results while on drug therapy. For example, plasma drug concentrations or liver biopsy where drug-induced hepatitis is suspected
- If new drug therapy is started which may be used to treat the symptoms of an ADR
- Listening to the patient’s own concerns regarding drug therapy.
How common are ADRs?
ADRs are common. A study of hospital admissions in the UK1 found that:
- 6.5% of admissions were related to ADRs
- projected annual cost to the NHS is £466 million
- over 2% patients admitted with an ADR died, suggesting an overall fatality rate from ADRs within the population of 0.15%
- 72% of ADRs were definitely or possibly avoidable.
1 Pirmohamed M et. al. Adverse drug reactions as cause of admission to hospital: Prospective analysis of 18820 patients. BMJ 2004;329;15-9.
