An MS flare-up, also called a relapse or exacerbation, is a period when multiple sclerosis causes new or worsening neurological symptoms that last at least 24 hours. It happens because the immune system launches a fresh attack on the protective coating around nerve fibers in the brain or spinal cord. Flare-ups are the hallmark of relapsing-remitting MS, the most common form of the disease, and they vary widely in severity, from mild sensory changes to significant loss of vision or mobility.
What Happens in Your Body During a Flare
MS is an autoimmune disease, meaning the immune system mistakenly targets healthy tissue. During a flare-up, immune cells cross into the central nervous system and attack myelin, the insulating sheath that wraps around nerve fibers. Myelin works like the coating on an electrical wire: it helps signals travel quickly and efficiently between the brain and the rest of the body. When that coating is stripped away (a process called demyelination), nerve signals slow down, get scrambled, or stop entirely.
This damage shows up on MRI scans as bright spots called lesions. During an active flare, a contrast dye injected before the scan can highlight areas of fresh inflammation, distinguishing new damage from older scars. If the inflammation resolves and myelin partially repairs itself, symptoms can improve. But repeated attacks in the same area can eventually damage the nerve fibers themselves, leading to permanent disability. That progression from temporary inflammation to lasting nerve degeneration is the central concern of MS treatment.
What a Flare-Up Feels Like
Symptoms typically develop over 24 to 48 hours and last anywhere from a few days to several weeks. About 80 to 100 percent of people see meaningful improvement afterward, though some residual effects may linger. The specific symptoms depend entirely on where in the central nervous system the new inflammation occurs.
Common flare-up symptoms include:
- Vision problems: partial or complete loss of vision in one eye, often with pain during eye movement, along with blurry or double vision
- Weakness: loss of strength in an arm or leg, sometimes making walking difficult or impossible
- Sensory changes: numbness, tingling, or a rising sense of numbness that starts in the legs and moves upward
- Balance and coordination issues: dizziness, vertigo, or unsteady gait
- Fatigue, bladder problems, or muscle spasms that are new or noticeably worse than baseline
A single flare doesn’t usually cause all of these at once. You might experience just one or two new symptoms, or you might notice that an old symptom from a previous attack has returned with greater intensity.
How Doctors Define a True Relapse
Not every worsening of symptoms counts as a genuine flare-up. The National Multiple Sclerosis Society defines a relapse as an episode that meets three criteria: it lasts at least 24 hours, it occurs at least 30 days after the last relapse, and it happens without an infection or other explanation. That last point matters because infections, fever, and other stressors can temporarily worsen MS symptoms without any new nerve damage occurring.
These temporary symptom flares are called pseudo-exacerbations, and they can feel identical to a real relapse. The most well-known type is the Uhthoff phenomenon, a transient worsening of neurological function triggered by a rise in core body temperature. Hot showers, exercise, sun exposure, fever, psychological stress, and even the menstrual cycle can all trigger it. The key difference is timing: pseudo-exacerbations resolve quickly once the trigger is removed, while true relapses persist and reflect new inflammatory activity in the nervous system.
If you’re unsure whether your symptoms represent a true flare, an MRI with contrast dye can help clarify the picture by showing whether there’s active inflammation.
Known Triggers for Relapses
Researchers have identified several factors that raise the likelihood of a genuine relapse. Stressful life events carry a moderate but consistent association with increased relapse risk. One analysis of extreme stress found that wartime conditions like missile attacks tripled the risk of relapse, though everyday stressors show a smaller effect.
The postpartum period is another well-documented trigger. In the first three months after delivery, the risk of relapse is roughly 87 percent higher than baseline. That risk drops back to normal, and even slightly below normal, by about 10 to 12 months after birth. This pattern is thought to relate to the dramatic immune system shifts that occur during and after pregnancy.
Infections have long been suspected as relapse triggers, and while systemic infections can activate inflammatory pathways, recent evidence suggests that disease-modifying therapies may blunt this effect. Notably, research on COVID-19 found no significant association between acute infection and MS relapse occurrence.
How Often Flare-Ups Happen
For people with relapsing-remitting MS who aren’t on disease-modifying therapy, clinical trial data from the 1990s showed annual relapse rates above 1.0, meaning roughly one or more relapses per year on average. More recent placebo-group data shows rates closer to 0.5 or below, a shift that may reflect changes in how trials select participants or evolving diagnostic criteria. Disease-modifying therapies reduce relapse rates further, which is one of the main reasons neurologists recommend starting treatment early.
Relapse frequency varies enormously between individuals. Some people go years between flare-ups, while others experience several within a single year. Higher relapse rates in the first few years after diagnosis are generally associated with faster disability accumulation over time.
How Flare-Ups Are Treated
Mild relapses that don’t interfere with daily function sometimes resolve on their own without specific treatment. For flare-ups that cause significant disability, the standard approach is a short course of high-dose corticosteroids, typically given intravenously over three to five days. The goal isn’t to reverse the underlying damage but to reduce inflammation quickly, shorten the duration of symptoms, and speed recovery.
Corticosteroids don’t work for everyone, and they come with side effects like insomnia, mood changes, a metallic taste, and fluid retention. When corticosteroids aren’t effective, aren’t tolerated, or are medically contraindicated, a procedure called plasmapheresis (plasma exchange) is sometimes used as a second-line option. This involves filtering the blood to remove the antibodies and inflammatory proteins that are attacking the nervous system.
It’s worth noting that treating a flare-up is different from treating MS overall. Corticosteroids address the acute inflammation of a single relapse. Disease-modifying therapies, taken on an ongoing basis, work to prevent future relapses from happening in the first place by keeping the immune system from launching new attacks on myelin. The two approaches complement each other but serve different purposes.
Recovery After a Flare
Most people recover substantially after a relapse, especially early in the disease. Symptoms often begin improving within days to weeks of the inflammation settling down, though full recovery can take months. Some relapses leave behind residual symptoms, a slight numbness or a bit of lingering fatigue that becomes part of your new baseline.
The degree of recovery depends on several factors: how much inflammation occurred, whether the nerve fibers themselves were damaged (not just the myelin), and where in the central nervous system the attack happened. Relapses affecting the optic nerve or spinal cord tend to produce more noticeable symptoms but also have fairly good recovery rates. Relapses that cause widespread or severe damage may leave more lasting effects. Rehabilitation, including physical therapy and occupational therapy, can help you regain function and adapt to any changes that persist after a flare resolves.