Chronic illness symptoms

ME/CFS, fibromyalgia, and Long Covid are three distinct conditions. They share a significant overlap in symptoms, which is part of why they are so often misdiagnosed, dismissed, or treated as interchangeable.

This page covers the symptoms that appear across all three. Where the conditions diverge, we flag the difference. For more detail on any one condition, follow the links at the end.

All three conditions involve the same broad systems: the brain, the immune system, the autonomic nervous system, and the body’s energy production. Research into how these systems interact is ongoing, and much of what is currently understood is still provisional.

In practice, a person with one condition often has symptoms that also belong to another. Some people meet the clinical criteria for more than one. Distinguishing between them requires a clinician — self-diagnosing from a symptom list is unreliable.

The fatigue in these conditions is not the same as being tired after a long day.

It is heavy, persistent, and not relieved by sleep. Many people describe it as a flu-like exhaustion that sits in the muscles and the mind at once. Everyday activities — washing, cooking, holding a conversation — can feel disproportionately costly.

Fatigue is a core symptom of ME/CFS and Long Covid. In fibromyalgia it is common but not the defining feature. Pain is.

A distinctive feature of ME/CFS is post-exertional malaise (PEM). PEM is a significant worsening of symptoms after physical, cognitive, or emotional effort, often delayed by 12 to 72 hours after the trigger.

A significant proportion of people with Long Covid also experience PEM, particularly those whose Long Covid meets ME/CFS criteria.

Fibromyalgia involves flares — periods of worsened symptoms triggered by stress, poor sleep, weather, or overexertion. Flares and PEM share surface similarities, but they are not the same phenomenon. Fibromyalgia flares do not follow the delayed-crash pattern that defines PEM.

Why the distinction matters

Treatments that worsen PEM — including graded exercise therapy — can cause long-term harm in ME/CFS. The 2021 NICE guideline (NG206) explicitly removed graded exercise therapy from ME/CFS care. Anyone managing symptoms across these conditions needs to know whether PEM is in the picture.

All three conditions commonly disrupt sleep in ways that extend far beyond ordinary insomnia.

People often report:

• Waking up feeling no better than when they went to bed
• Difficulty falling asleep despite exhaustion
• A sleep schedule that drifts later and later
• Vivid, disturbed, or unusually intense dreams
• Waking multiple times during the night

Sleep studies often come back without clear findings. This does not mean the experience is not real.

Pain is present in all three conditions, but with different profiles.

• In fibromyalgia, widespread body pain is the defining feature. It is often described as aching, burning, or a sensation of pressure on the muscles.
• In ME/CFS, pain is common and includes muscle aches, joint pain, and new types of headache — but it is not required for diagnosis.
• In Long Covid, pain varies widely. Headaches, muscle pain, and chest pain are all frequently reported, but the pattern differs considerably between individuals.

Over-the-counter painkillers are often less effective for this kind of pain than they are for injury-related pain. The NICE guideline on chronic primary pain (NG193) covers some of this territory, but is contested within the fibromyalgia community, particularly around its recommendations on analgesics.

Brain fog is an umbrella term for a cluster of cognitive difficulties that appear across all three conditions. It can include:

• Losing words mid-sentence
• Difficulty following a conversation
• Forgetting things that happened moments earlier
• Slow, effortful thinking that feels like moving through resistance
• Trouble switching between tasks
• Difficulty reading for more than short periods

Brain fog is one of the most disabling aspects of these illnesses, and one of the least visible to others.

Orthostatic intolerance — symptoms that worsen on sitting or standing, and improve on lying flat — is common in all three conditions. Postural tachycardia syndrome (POTS) is a recognised subtype, where the heart rate rises sharply on standing.

Common signs include:

• Dizziness or light-headedness on standing
• A racing or pounding heart
• Nausea when upright
• Fainting or near-fainting
• Feeling worse toward the end of the day, after more time spent upright

Orthostatic intolerance is particularly well-documented in ME/CFS and Long Covid. It is less commonly highlighted in fibromyalgia, but not absent from it.

Light, sound, smell, and touch can all feel overwhelming during flares or crashes. People with ME/CFS and Long Covid at the more severe end of the spectrum sometimes spend long periods in darkened rooms to reduce sensory load.

This is not a preference. It reflects a genuine change in how the nervous system processes input.

The list below is not exhaustive. Long Covid in particular affects more body systems more variably than the other two, and no symptom list can capture the full range.

Body-system symptoms

• Temperature dysregulation. Night sweats, chills, or difficulty tolerating heat or cold.
• Digestive symptoms. Nausea, IBS-like patterns, or new food intolerances.
• Heart symptoms. Palpitations, chest pain, or unexplained tachycardia.
• Skin symptoms. Rashes, flushing, or changes in skin sensitivity.

Immune and nerve symptoms

• Immune-style symptoms. Sore throat, tender lymph nodes, or low-grade fevers.
• Nerve symptoms. Numbness, tingling, twitching, or pins and needles.
• Menstrual symptoms. Worsening of symptoms around periods, or changes in cycle.
• New sensitivities. To medications, alcohol, or foods that were previously fine.

Symptoms in all three conditions tend to fluctuate — day to day, week to week, and often season to season. A tracking method of some kind is the most reliable way to see patterns.

A paper diary is often enough. Apps such as Visible and Bearable add more detail if that is useful. See our ME/CFS Resources for more information about tracking symptoms.

Many of the symptoms described here have multiple possible causes. Before a chronic illness can be diagnosed, doctors need to rule out other conditions with overlapping symptoms. This is part of why the diagnostic process can take months or years.

Keeping a record of your symptoms, their triggers, and their duration gives a clearer picture for that conversation. Our guide to ME/CFS diagnosis covers how to prepare for an appointment when brain fog is in the mix — most of the guidance applies across the three conditions.

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