Overview
Chronic Fatigue Syndrome (CFS), also called Myalgic Encephalomyelitis (ME), is a complex, long-term condition characterised by extreme fatigue that is not explained by other conditions and does not improve with rest. Even minor physical or mental exertion can cause a worsening of symptoms (post-exertional malaise) that can last days.
How common is it?
ME/CFS can affect people of any age, including children. It is more commonly diagnosed in women.
Causes and risk factors
The exact cause is unknown but ME/CFS often begins after a viral infection. Research points to abnormalities in immune function, energy production in cells, and the autonomic nervous system.
Common risk factors
- Viral infection (often appears after glandular fever, COVID-19)
- Female sex
- The causes and individual risk factors remain incompletely understood
- Symptoms can begin after an infection or develop gradually without an obvious trigger
- Possible genetic susceptibility
- Immune and energy-processing changes are being researched; they are not established screening markers
Symptoms
- Severe fatigue not relieved by sleep or rest
- Post-exertional malaise: worsening after even mild activity
- Unrefreshing sleep
- Cognitive difficulties: poor memory, concentration ('brain fog')
- Muscle and joint pain
- Headaches
- Sore throat and enlarged lymph nodes
- Dizziness when standing (orthostatic intolerance)
When to see a doctor
See a doctor if you have persistent, unexplained fatigue lasting more than 4 weeks that significantly limits your daily activities. Blood tests are needed to rule out treatable causes such as anaemia or thyroid disease.
Diagnosis
Diagnosis is clinical, based on specific criteria (the Canadian Consensus Criteria or NICE 2021 guidelines) after excluding other causes. Blood tests, thyroid function, and sleep assessment are part of the workup.
Treatments
Pacing and energy management
Keep physical, cognitive, emotional and social activity within current energy limits, with planned rest. Any change in activity should be individualised and flexible, only after stability, and adjusted down as well as up. Do not use programmes with fixed increases in exercise or activity.
Sleep management
Establishing regular sleep patterns and treating sleep disorders (if present) can reduce fatigue severity.
Symptom management
Clinicians may offer treatment for specific symptoms, such as pain, sleep problems or orthostatic intolerance, taking account of individual medicine sensitivity. There is no medicine that cures ME/CFS.
Self-care and lifestyle
- Keep an activity diary to identify your energy limits and patterns
- Prioritise essential activities when energy permits, with rest and flexibility within your limits
- Rest proactively, before fatigue hits, not only in response to it
- Connect with ME/CFS support organisations for practical guidance
Prevention
There is no proven way to prevent ME/CFS. If symptoms develop after an infection, seek assessment and avoid pushing through exhaustion or post-exertional worsening.
Sources for targeted updates
Updated: 24 October 2026. Targeted corrections to risk factors, treatment, self-care, prevention, frequency. This update covers the named sections; a full review of the remaining guide is pending.