Symptom cluster
Fatigue and post-exertional malaise
Exhaustion that rest doesn't fix, and crashes that follow activity by hours or days. This page covers: Post-exertional malaise (PEM/PESE), Chronic fatigue, Myalgia, Exercise intolerance / reduced VO2.
What this is
This is exhaustion that rest does not fix, paired with its defining feature: post-exertional malaise (PEM) — a delayed crash, often hours to a day or two after activity, where symptoms flare and your function drops.
The trigger can be physical, mental, or emotional, and the “payback” is out of proportion to the effort. PEM is the central reason ordinary exercise advice can backfire here: pushing through tends to make people worse, not stronger.
This pattern overlaps closely with ME/CFS, and learning to stay within your energy limits — pacing — is the safety-critical idea that runs through this whole section.
How to read this page
Each row under What causes it and What has been tried represents one proposed explanation or treatment. The tags help you judge the claim quickly, without treating every row as equally proven.
gradeHow strong the underlying evidence is.
Example: a treatment tested in multiple randomized controlled trials would receive a stronger grade than one supported only by case reports, expert opinion, or animal studies.
auditHow reliable the claim remains after tracing its sources.
Example: a claim may appear in many reviews, but if those reviews all cite the same small, uncontrolled study, the audit may be weak because the claim is being repeated more than it is being independently confirmed.
A claim can have a strong grade but a weak audit, or the reverse. That is why both are shown.
endpointWhat was actually measured.
Example: a study showing that patients “felt better” measured a patient-reported outcome. A study showing improved oxygen levels, reduced inflammation markers, better walking distance, or abnormal MRI findings measured something different. The endpoint tells you what kind of evidence the claim rests on.
safety / conflictImportant cautions about harm, bias, or financial interests.
Example: a supplement may look low-risk but still interact with medications. A device or drug study may look promising, but if the evidence comes mainly from the manufacturer, that conflict should be visible before the claim is weighed.
What causes it
The leading explanations, each with its evidence grade and audit status. Several are shared with other clusters.
caused byPost-exertional malaise (PEM/PESE)
Mitochondrial / metabolic dysfunction produces Post-exertional malaise (PEM/PESE).
caused byChronic fatigue
EBV / herpesvirus reactivation produces Chronic fatigue.
caused byExercise intolerance / reduced VO2
Mitochondrial / metabolic dysfunction produces Exercise intolerance / reduced VO2.
Deconditioning (as cause) contested-cause Exercise intolerance / reduced VO2.
Dysautonomia / autonomic dysfunction produces Exercise intolerance / reduced VO2. full page →
What has been tried
No treatment is approved. Each row shows how strong the evidence is, what outcome it was measured on, and any safety or conflict flag. Note that a drug can help a lab number without helping how you feel.
tried forPost-exertional malaise (PEM/PESE)
Pacing / energy-envelope management manages Post-exertional malaise (PEM/PESE). full page →
Graded exercise therapy (GET) harms Post-exertional malaise (PEM/PESE).
tried forChronic fatigue
Oxaloacetate (anhydrous enol) treats Chronic fatigue.
Low-dose naltrexone treats Chronic fatigue. full page →
Metformin treats Chronic fatigue.
Fluvoxamine / SSRI treats Chronic fatigue.
H1/H2 antihistamines (fexofenadine, famotidine) relieves Chronic fatigue.
tried forExercise intolerance / reduced VO2
Pyridostigmine improves Exercise intolerance / reduced VO2.
Recumbent / supine-biased rehabilitation (PEM-bounded) manages Exercise intolerance / reduced VO2.
How it is measured
tests forPost-exertional malaise (PEM/PESE)
2-day cardiopulmonary exercise test (CPET) indicates Post-exertional malaise (PEM/PESE).
tests forExercise intolerance / reduced VO2
Invasive CPET (iCPET) diagnoses Exercise intolerance / reduced VO2.
A new 399-person trial suggests fluvoxamine reduces fatigue. The tracker has proposed strengthening that claim; until a reviewer accepts it, the page above shows the current audited state, not the proposal.