ME/CFS and Post-Exertional Malaise

What ME/CFS is

Myalgic encephalomyelitis, also called chronic fatigue syndrome (ME/CFS), is a long-term condition that can affect different parts of the body. The most common symptom is extreme tiredness. The cause is unknown. [1] This page is educational. It is not a way to decide whether you have ME/CFS, because similar symptoms occur in many other illnesses. [2]

What post-exertional malaise (PEM) means

PEM means symptoms get worse after physical or mental activity. It can be triggered by things like socializing or studying, can start hours or days after the activity, and can take weeks to recover from. [2] NICE describes PEM as often delayed in onset by hours or days, disproportionate to the activity, and with a prolonged recovery. [3]

PEM is different from ordinary tiredness after effort. This is why this page does not give general advice to "build up" activity. See Why am I so tired?.

The main features clinicians look at

NHS describes four main symptoms: extreme tiredness that does not go away with rest or sleep, sleep problems, problems with thinking, concentration and memory ("brain fog"), and PEM. [1] [2] Other symptoms can include pain, headaches, a sore throat, dizziness on standing and sensitivity to light or sound. [2] For related thinking and memory symptoms, see Brain fog.

How it is assessed

There is no specific test. ME/CFS is diagnosed from symptoms and by ruling out other conditions. [1] NICE says to explain that no diagnostic test exists, and to carry out a medical assessment and investigations to exclude other diagnoses, such as a full blood count, thyroid function, ferritin and screening for celiac disease. [3] This is why other causes of fatigue, such as anemia or thyroid problems, are looked for first.

Care and pacing

There is currently no cure, but treatments may help manage symptoms. [1]

• Energy management (often called pacing). NICE describes it as a self-management approach that is not curative, covers all kinds of activity (physical, mental, emotional and social), and helps people use the energy they have while reducing the risk of PEM by exceeding their limits. Activity is not automatically increased; it is maintained or adjusted up or down. [3]
• Exercise. NICE says a physical activity program should be considered only for people who feel ready or want one, should be overseen by a physiotherapist in an ME/CFS specialist team, and should start from a baseline that does not worsen symptoms. It recommends against programs with fixed incremental increases such as graded exercise therapy, and against any exercise offered as a cure. [3] NHS likewise says graded exercise therapy is not recommended. [4]
• CBT. NICE says cognitive behavioral therapy should only support people to manage symptoms and should not be offered as a cure. [3]
• Food and supplements. NHS advises regular, balanced meals and says there is insufficient evidence to recommend supplements such as vitamin B12, vitamin C, magnesium or coenzyme Q10. [4]
• Setbacks. Relapses are common and may follow an infection or unplanned activity. [4]

Where sources differ

Evidence: mixed. A Cochrane review (published in 2019, searches to 2014, using older diagnostic criteria) found exercise therapy probably reduced fatigue at the end of treatment compared with passive control, but all included trials had a high risk of performance and detection bias, and the evidence on harms was very uncertain. [5] NICE's 2021 guideline weighed the evidence differently and recommends against graded exercise. [3] The US MedlinePlus entry says a care team helps work out how much activity you can do. [6] Because sources differ, follow individual advice from a clinician experienced in ME/CFS.

Long COVID and PEM

NHS says long COVID is when symptoms last longer than 12 weeks, and extreme tiredness is among the most common. It advises people not to restart exercise before talking to a GP or specialist, and to be careful and stop if symptoms appear. [7] A 2025 meta-analysis estimated PEM in about 25% of community adults with post-COVID condition (low certainty), and all included studies had high or very high risk of bias. [8] NICE says after COVID-19 blood tests such as full blood count, ferritin and thyroid function may be offered depending on symptoms. [9]

When to get help

If tiredness lasts, disrupts daily life, or gets worse after activity, see a GP. [2] If symptoms are severe, NHS says a specialist should be involved. [2] If you have thoughts of harming yourself, call or text 988 in the US (free, 24/7), or use your local emergency number. Read Fatigue: when to get help and Fatigue from medicines, mood and other conditions.