Medical Matters > Symptoms: Post-Exertional Malaise (PEM)

ME Essential Summer 2023


What is Post-Exertional Malaise and how can it best be managed? 


In ME/CFS, even the smallest amount of physical, mental, emotional, or social activity can trigger post-exertional malaise (PEM), resulting in a worsening of symptoms, and leading to a reduction in functional ability. It is not always apparent when PEM might occur which makes it one of the more difficult symptoms to recognise and manage. The worsening of symptoms is often delayed by hours or days and relative improvement can take much longer to achieve. 

PEM is one of 4 key symptoms necessary for a diagnosis of ME/CFS (and has also emerged as a key symptom in people diagnosed with Long Covid). The following 4 symptoms (as defined by the 2021 NICE Guideline on ME/CFS) must be present for a minimum of 6 weeks when considering a diagnosis of ME/CFS and should account for a significant decline in pre-illness levels of functional ability: 

  1. Debilitating fatigue that is worsened by activity, is not caused by excessive cognitive, physical, emotional, or social exertion, and is not significantly relieved by rest.  
  1. Post-exertional malaise after activity in which the worsening of symptoms:  
    - is often delayed in onset by hours or days.  
    - is disproportionate to the activity.  
    - has a prolonged recovery time that may last hours, days, weeks or longer.  
  1. Unrefreshing sleep or sleep disturbance (or both), which may include:  
    - feeling exhausted, feeling flu-like and stiff on waking.  
    - broken or shallow sleep, altered sleep pattern or hypersomnia.  
  1. Cognitive difficulties (sometimes described as ‘brain fog'), which may include problems finding words or numbers, difficulty in speaking, slowed responsiveness, short-term memory problems, and difficulty concentrating or multitasking. 

The 2021 NICE Guideline summarises Post-exertional malaise as: 

“The worsening of symptoms that can follow minimal cognitive, physical, emotional, or social activity, or activity that could previously be tolerated. Symptoms can typically worsen 12 to 48 hours after activity and last for days or even weeks, sometimes leading to a relapse. Post-exertional malaise may also be referred to as post-exertional symptom exacerbation.” 

The NICE Guideline recommends Energy Management


Also see the section on care for people with severe or very severe ME/CFS.

Discuss with people with ME/CFS the principles of energy management, the potential benefits and risks and what they should expect. Explain that it:

  • is not curative.
  • is a self-management strategy led by the person themselves with support from a healthcare professional in an ME/CFS specialist team.
  • includes all types of activity (cognitive, physical, emotional and social) and takes into account overall level of activity.
  • helps people learn to use the amount of energy they have while reducing their risk of post-exertional malaise or worsening their symptoms by exceeding their limits.
  • recognises that each person has a different and fluctuating energy limit and they are experts in judging their own limits.
  • can include help from a healthcare professional to recognise when they are approaching their limit (children and young people in particular may find it harder to judge their limits and can overreach them).
  • uses a flexible, tailored approach so that activity is never automatically increased but is maintained or adjusted (upwards after a period of stability or downwards when symptoms are worse).
  • is a long-term approach − it can take weeks, months or sometimes even years to reach stabilisation or to increase tolerance or activity.

More information


Information provided by The ME Association should not be construed as medical advice. Don't assume any new or worsened symptoms are simply the result of having ME/CFS or Long Covid. We recommend that any information you deem relevant is discussed with your NHS GP as soon as possible. It is important that you seek personalised medical advice from the GP who is in charge of your care and who knows you well.

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