The short version
ME/CFS stands for myalgic encephalomyelitis / chronic fatigue syndrome. It’s a long-term illness that affects how the body makes and uses energy, as well as the immune system and the nervous system.
The name “chronic fatigue syndrome” has done people a disservice. The fatigue in ME/CFS isn’t ordinary tiredness that a good night’s sleep or a holiday fixes. People describe it as feeling poisoned, flu-like, or “running on a battery that never charges past 20%”.
The defining feature: post-exertional malaise (PEM)
The one symptom that sets ME/CFS apart from other fatiguing illnesses is post-exertional malaise, or PEM.
When someone with ME/CFS does more than their body can manage that day, they crash. The crash:
- usually arrives 12–72 hours later, not straight away, which makes the cause easy to miss
- can last days, weeks or longer
- can bring on flu-like symptoms, pain, brain fog, poor sleep and profound weakness
- can be triggered by physical, mental or emotional effort. A long conversation, a busy social evening, a stressful appointment or a shower can all be enough.
Repeated crashes can push someone into a worse, longer-lasting state. That’s why pacing matters so much.
Other core features
- Unrefreshing sleep: waking up feeling as tired as when you went to bed, however long you slept
- Cognitive problems (“brain fog”): trouble thinking, finding words, concentrating or remembering
- Orthostatic intolerance: feeling worse when upright, e.g. dizziness, a racing heart, or needing to sit or lie down. See Dizziness on standing & POTS.
- Pain: muscle and joint pain, headaches
- Immune-type symptoms: sore throat, tender lymph nodes, feeling “like I’m coming down with something”
- Sensory sensitivity: to light, sound, smell or touch
See the full list on Symptoms.
What ME/CFS is not
- It is not depression or anxiety. People with ME/CFS can also feel low or anxious, as anyone with a serious chronic illness might. But ME/CFS is a physical illness with measurable biological changes. See The science.
- It is not deconditioning. Being unfit doesn’t cause PEM, and exercise programmes don’t cure ME/CFS. Pushing through makes it worse.
- It is not “all in your head”. Brain imaging, blood flow studies, immune cell studies and a large genetics study have all found real differences.
Why routine tests come back normal
Standard blood tests look for things like anaemia, infection, inflammation, and thyroid or kidney problems. In ME/CFS these are usually normal. The problems researchers have found (in immune cell ion channels, blood flow to the brain when upright, how muscles make energy) aren’t measured by a standard GP blood panel.
A normal result is still useful: it helps rule out other conditions that can look like ME/CFS. See Tests to ask your GP for.
Can people recover?
Some people improve substantially, especially if the illness is recognised early and they pace carefully. Others stay unwell for many years, and a significant minority become severely unwell. People can also improve and later relapse, often after a new infection or a period of overload.
Is it the same as Long COVID?
They overlap a lot. Many people with Long COVID meet the criteria for ME/CFS, and the two illnesses share many biological findings. Because Long COVID has attracted far more research funding, a lot of the trials that may help people with ME/CFS are actually Long COVID trials. We include them on our trials page where they’re relevant.
Is it the same as POTS, fibromyalgia or “chronic fatigue”?
No, although they often overlap. See ME, CFS, POTS & related conditions.
Where to next?
- Symptoms: the full picture, and how ME/CFS is diagnosed
- Causes & triggers: what sets it off, and who gets it
- Pacing: the one thing every expert agrees on