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What is an adverse reaction to a medicinal product?

  • An adverse reaction is an unintended or adverse response following the administration of one or more medicinal products under normal conditions of use, suspected to have arisen in correlation with the administration of the medicine.
  • An adverse reaction may be a known side effect of the treatment or it may involve a previously unknown adverse reaction.

Definitions of adverse reactions stipulated by Act No 378/2007 Coll. on Pharmaceuticals and on Amendments to Some Related Acts (Act on Pharmaceuticals):

Adverse reaction to a medicinal product shall mean an adverse and unintended response to the product administration which occurs at doses normally used for the prophylaxis, therapy or diagnosis of a disease or for the restoration, correction or other modification of physiological functions. This definition shall not apply to transfusion products. (Section 3, paragraph 4)

Serious adverse reactions are those which result in death, are life-threatening, require hospitalisation or prolongation of existing hospitalisation, result in persistent or significant disability or incapacity or are demonstrated as a congenital anomaly or birth defect in offsprings. (Section 3, paragraph 4, letter a))

Unexpected adverse reactions are those the nature, severity or consequences of which are not consistent with the information laid down in the summary of the product characteristics for an authorised medicinal product or which are not consistent with available information, e.g. the investigator's brochure for an investigational medicinal product without marketing authorisation. (Section 3, paragraph 4, letter b))


Type A adverse reactions

Type A adverse reactions (escalating) occur as a consequence of excessive normal pharmacological effect of the medicine in a normally applied dose. Type A adverse reactions are dose-dependent.

For example:

  • Low blood pressure following the administration of antihypertensive products
  • Low blood sugar levels following the administration of insulin

Type A adverse reactions include, furthermore, reactions which are not associated with the intended pharmacological effect (such as dry mouth following the administration of tricyclic antidepressants).  

Type B adverse reactions

Type B adverse reactions cannot be deduced from the known pharmacological effect of the medicine.

For example:

  • Anaphylaxis following the administration of penicillin
  • Rash following the administration of antibiotics


Characteristics of Type A and Type B adverse reactions

Type A Type B



Usually dose-dependent  

Rarely dose-dependent

High morbidity

Low morbidity

Low mortality

High mortality

Respond to dose reduction Respond only to discontinuation of the medicine


Drug safety, Reprint collection 2001, Adis international 2001, New Zealand, An ABC of drug-related problems

Which adverse reaction is serious?

A serious adverse reaction is any adverse reaction (i.e. also an adverse reaction listed in the summary of the product characteristics) which results in the following:

  • death of the patient;
  • life-threatening condition of the patient;
  • significant disability;
  • permanent consequences;
  • hospitalisation or prolongation thereof;
  • congenital anomaly in offsprings;
  • other serious consequences, which may include e.g. also incapacitation