An MS relapse happens when new inflammation in the brain or spinal cord causes new neurological symptoms or makes existing MS symptoms significantly worse. A true relapse usually:
- Develops over hours or days
- Lasts for more than 24 hours
- Occurs after symptoms have been stable for at least a month
- Is not explained by an infection, fever or another temporary trigger
Some relapses are mild and improve without treatment. Others can affect vision, walking, strength or daily activities and may require corticosteroids or rehabilitation.
*AI-generated image - for illustration only. Clinical accuracy is not guaranteed.
For a broader overview of long-term therapies, see our main guide to Multiple Sclerosis (MS) Treatment.
What Is Happening Inside the Body During an MS Relapse?
Multiple sclerosis is an immune-mediated disease of the central nervous system. During a relapse, increased immune activity causes inflammation in an area of the brain, spinal cord or optic nerves. This inflammation can damage myelin, the protective covering around nerve fibres and interfere with the electrical signals travelling along those nerves.
The symptoms that appear depend on where the inflammation occurs. For example:
- Inflammation affecting the optic nerve may cause blurred or reduced vision
- Inflammation in the spinal cord may cause weakness, numbness or bladder problems
- Inflammation affecting balance pathways may cause dizziness or poor coordination
What Does an MS Relapse Feel Like?
A relapse may cause an entirely new symptom or bring back a neurological problem that had previously improved. Common relapse symptoms can include:
- Blurred or reduced vision, sometimes with pain when moving the eye
- New numbness or tingling
- Weakness in an arm or leg
- Difficulty walking
- Poor balance or coordination
- Double vision
- Increased muscle stiffness or spasms
- Changes in bladder function
*AI-generated image - for illustration only. Clinical accuracy is not guaranteed.
How Does an MS Relapse Usually Progress?
A relapse often develops gradually rather than appearing instantly.
| Stage | What May Happen |
|---|---|
| Beginning | New symptoms appear or old symptoms become noticeably worse |
| Next few days | Symptoms may increase as inflammation develops |
| Plateau | Symptoms may remain relatively stable for a period |
| Recovery | Symptoms gradually begin improving over days, weeks or sometimes months |
How Long Does an MS Relapse Last?
To qualify as a relapse, symptoms generally need to last longer than 24 hours. In reality, many relapses last:
- Several days
- A few weeks
- Occasionally several months before recovery stabilises
Recovery is often most noticeable during the first weeks and months. The exact recovery time depends on the location and severity of inflammation and how much neurological damage occurred.
Is It a True MS Relapse or a Pseudo-Relapse?
Not every temporary worsening of MS symptoms means new disease activity. A pseudo-relapse happens when an old MS symptom temporarily becomes worse because damaged nerves are struggling under additional stress. Common triggers include:
- Urinary or respiratory infection
- Fever
- Hot weather or overheating
- Extreme fatigue
- Stress
Once the trigger is removed—for example, an infection is treated or body temperature returns to normal—the symptoms usually improve. A pseudo-relapse does not necessarily mean that new inflammation or a new MS lesion has developed.
*AI-generated image - for illustration only. Clinical accuracy is not guaranteed.
Why Do Doctors Check for Infection First?
Infections can temporarily make existing MS symptoms much worse and can easily be mistaken for a relapse. Urinary tract infections and respiratory infections are particularly important to rule out. Before confirming a relapse or prescribing high-dose steroids, the MS team may therefore ask about:
- Fever
- Urinary symptoms
- Cough or respiratory symptoms
- Other signs of infection
Does Every MS Relapse Need Steroids?
No. Mild relapses that have little effect on daily activities may improve naturally without corticosteroids. Current NICE guidance recommends considering treatment when a relapse significantly affects normal activities such as:
- Walking
- Using the hands
- Working
- Seeing clearly
- Managing everyday tasks
*AI-generated image - for illustration only. Clinical accuracy is not guaranteed.
Does a Relapse Mean MS Treatment Has Failed?
Not automatically. No disease-modifying therapy prevents every relapse. However, a relapse—particularly when accompanied by new MRI activity—may indicate that MS is not adequately controlled.
The neurologist may review:
- How often relapses are occurring
- Whether new MRI lesions are appearing
- How severe the relapse was
- Whether the current medicine is being taken correctly
- Whether a stronger or different DMT should be considered
Relapse or MS Progression: What Is the Difference?
An MS relapse usually causes a relatively noticeable change over hours or days, followed by some degree of recovery. MS progression means neurological function gradually worsens over a longer period without a clear attack and recovery pattern. This distinction matters because the treatment approach can be different.
The Bottom Line
During an MS relapse, new inflammation develops in the central nervous system and temporarily disrupts the way nerve signals travel. This can cause new neurological symptoms or a significant worsening of previous symptoms.
A true relapse generally lasts more than 24 hours, follows at least a month of relatively stable symptoms and is not caused by infection, fever or another temporary trigger. Not every relapse requires steroids. Mild attacks may recover naturally, while relapses affecting important daily activities can be treated with high-dose corticosteroids to speed recovery.
Severe relapses that do not respond adequately to steroids may occasionally require plasma exchange. Most importantly, a new relapse should be reported to the MS team because it can provide important information about whether the current long-term MS treatment is controlling the disease effectively.
