Skip to content


Post Exertional Malaise (PEM) is the hallmark symptom of ME/CFS

It is the worsening of symptoms following even minor exertion. This exertion could be physical, cognitive, or emotional

Any activity which causes a person with ME/CFS to exceed their energy limit can lead to PEM. This could be as simple as:

Walking or other light exercise
Attending a social event
Reading or writing
Exposure to excess stimulus (e.g. light/sound)
Taking a shower or bath

The onset of PEM is typically delayed by 12-72 hours following the activity.

It causes a worsening of symptoms for a period of time, this is often referred to as a “flare” or a “crash”

It can take days, weeks, or months to return to previous levels of functioning (or “baseline”).

It is very common, particularly in non diagnosed or newly diagnosed cases of ME/CFS, to see repeated cycles of over activity and crashing.

This is often referred to as the “push and crash” cycle.

Over time this can lead to extended periods of PEM, or even permanent reduction in level of functioning