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Pharmacology and Therapeutics

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Glossary of Pharmacology 

Side Effect

A side effect is a type of Adverse Drug Reaction (ADR) that is defined as a secondary effect of a drug, which may have beneficial or harmful consequences.  They are often due to off-target effects of the drug, for example on different organs or tissues than the target organ.  This term was first used in the modern concept in 1814.  A side effect is a predictable type of Adverse Drug Reaction, notably for off-target effects which can be predicted by the effect of the drug on tissues and organs other than the therapeutic target.  Side effects typically occur when the drug is used in the correct dose and by the right route.  They are considered to be a Type A or Predictable Adverse Drug Reaction. 

Side effects are one of the commonest types of Adverse Drug Reactions and also typically the least severe forms of ADRs The assessment and management of side effects are an important skill set for clinicians, which can include changing therapy or in other cases continuing therapy while the side effect diminishes and for some drugs vanishes over time.  This points to the importance of health care providers being well versed in the pharmacology of the drugs that they use.  Anticipatory guidance for predictable side effects increases patient confidence and empowerment and enhances likelihood of adherence to therapy.   

As well, understanding the mechanism of how side effects occur provides key insights into how drugs work on a mechanistic basis.  This can help determine the therapeutic approach and also can provide insights for certain drugs as to what might be novel or new indications. 

Examples of Side Effects: 


Side Effect 





Suppression of normal bowel flora by the antibiotic 

Maintain hydration 

May require change of antibiotic to one with less of a risk of diarrhoea 


Fine hand tremor 

Off-target activation of beta receptors on skeletal muscle 

Treat through; the tremor will diminish with continuing therapy 



Receptor-driven decrease in bowel tone 

Increasing fluid and fiber intake 



Suppression of kininase II activity and enhanced cough reflex 

Discontinuation of therapy, use of alternate treatment 

Teaching tips:  

Considerations around Side Effects include: 

  • Understanding mechanisms of Side Effects as the result of off-target effects 

  • Describing how to anticipate Side Effects based on understanding the mechanism of drug action and how to develop strategies to monitor for potential Side Effects and anticipatory management 

  • Communicating clearly with patients and families as to what Side Effects to anticipate and how to deal with a potential Side Effect 

  • Developing a plan to manage Side Effects including the impact on future therapy and the health care provider-patient relationship  

Suggested articles on this topic:
      • Edwards IR.  Adverse drug effects and their clinical management:  A personal view.  Drug Safety 2014; 37: 383-90. 
        - This manuscript reviews approaches to the management of common side effects. 

        - This is a US FDA website providing information on side effects to patients. 

      Linked terms: Adverse Drug Effect, Drug Safety, Pharmacovigilance, Drug Allergy  

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