Why do people with MS feel worse in the heat?

Heat slows or blocks conduction in demyelinated nerve fibers, so signals that get through at a normal temperature stop getting through when the person warms up. This temporary worsening, called Uhthoff phenomenon, affects an estimated 60 to 80% of people with MS and usually reverses within 24 hours as body temperature falls.

How small a temperature change matters?

A rise in core body temperature of as little as about 0.5 °C can be enough to trigger temporary symptoms in heat-sensitive MS. Triggers include hot weather, hot baths, fever, and exercise.

Does MS reduce sweating?

It can. MS lesions can disrupt the pathways that control sweating, leading to reduced or regional loss of sweating and, in advanced disease, more widespread loss. Across studies, pathologic sweating-test results have been reported in 26 to 94% of MS patients.

Is heat sensitivity in MS dangerous or permanent?

Uhthoff phenomenon is temporary and reverses when body temperature returns to baseline; it reflects reversible conduction changes, not new damage. Reduced sweating is a separate issue that can limit cooling and may relate to disability severity. Any concerns should be discussed with the treating clinician.

Can sweating problems and heat sensitivity make each other worse?

Yes. Sweating is the body's main way of shedding heat by evaporation. If MS reduces sweating, core temperature rises faster, which makes the temperature-triggered worsening of Uhthoff phenomenon more likely.

Is this page medical advice?

No. It is a summary of verified figures from published sources. Only a clinician can diagnose MS or determine whether temperature-related symptoms relate to it.

Research

Multiple Sclerosis, Heat Sensitivity, and Sweating

Multiple sclerosis (MS) damages the myelin that insulates nerve fibers in the central nervous system, which slows or blocks nerve conduction. Two consequences involve temperature. First, most people with MS are heat sensitive: a small rise in body temperature can temporarily worsen symptoms, a pattern called Uhthoff phenomenon that affects an estimated 60 to 80% of patients. Second, MS lesions can disrupt the pathways that drive sweating, so some patients have reduced sweating, which further limits the body's ability to cool itself. This page summarizes what the peer-reviewed literature reports. It is educational only and is not a diagnosis or medical advice.

By the Sweat Explained Editorial Team · Published 2026-07-20 · Last reviewed 2026-07-20 · Educational information, not medical advice.

Key statistics at a glance

60 to 80%
of people with MS experience temporary symptom worsening with heat (Uhthoff phenomenon)
Christogianni 2018 (Temperature); Davis 2018
~0.5 °C
a core-temperature rise this small can be enough to trigger temporary symptom worsening in heat-sensitive MS
Christogianni 2018 (Temperature)
< 24 hours
Uhthoff symptoms are typically short-lived and reverse as body temperature returns to baseline
Panginikkod StatPearls
26 to 94%
reported range of MS patients with pathologic sweating (sudomotor) test results across studies
Pintér 2015 (Int J Mol Sci)
~2.8 million
estimated number of people living with MS worldwide
Walton 2020 (Mult Scler, Atlas of MS)

Why heat worsens MS symptoms

In MS, the loss of myelin leaves demyelinated axons unusually vulnerable to temperature. As body temperature rises, conduction along these fibers slows further and can block entirely, so signals that were getting through at a normal temperature stop getting through when the person is warm. Reviews estimate that 60 to 80% of people with MS experience this temporary worsening with heat, known as Uhthoff phenomenon (Christogianni et al., 2018; Davis et al., 2018).

The trigger threshold is low. A rise in core temperature of as little as about 0.5 °C can be enough to bring on symptoms such as blurred vision, fatigue, weakness, or worsened coordination. Common triggers include hot weather, hot baths or showers, fever, and exercise. The effect is generally temporary: symptoms reverse, usually within 24 hours, once body temperature returns to baseline. Uhthoff phenomenon reflects reversible conduction changes, not new or permanent damage.

What commonly raises body temperature

Because heat sensitivity is driven by core temperature, a range of everyday exposures can provoke temporary symptoms in heat-sensitive MS:

Warm environments: hot weather, humid conditions, saunas, and hot rooms.
Hot water: hot baths and showers. The historical hot-bath test for MS was based on this sensitivity.
Fever and illness: any cause of raised core temperature, including infection.
Physical exertion: exercise raises metabolic heat production, which is why Uhthoff first described exercise-related visual changes.
Impaired cooling: when sweating is reduced (see below), the body warms faster and symptoms can appear sooner.

How MS can affect sweating

MS lesions can occur in the brain and spinal-cord regions that control body temperature and the sudomotor pathways that drive sweating. When those pathways are involved, the sweating response is blunted or lost, most often as reduced or regional loss of sweating (hypohidrosis or anhidrosis), and total loss of sweating can appear in advanced disease (Davis et al., 2018).

Formal testing confirms this is common but variable. A comprehensive review of autonomic dysfunction in MS reported that pathologic sweating-test results (using methods such as the thermoregulatory sweat test, sympathetic skin response, and QSART) were found in 26 to 94% of patients across studies, with the wide range reflecting different testing methods and definitions rather than disagreement about whether sudomotor dysfunction occurs (Pintér et al., 2015). Impaired sweating in MS has also been correlated with the severity of overall clinical disability.

Two temperature-related problems in MS

MS affects temperature in two distinct but linked ways. Distinguishing them helps clarify why heat is such a consistent theme in the MS literature.

Heat sensitivity versus impaired sweating in MS
Feature	Heat sensitivity (Uhthoff)	Impaired sweating (sudomotor)
What it is	Temporary symptom worsening when body temperature rises	Reduced or absent sweating from disrupted sudomotor pathways
Mechanism	Slowed or blocked conduction in demyelinated axons	Central lesions affecting thermoregulatory and sudomotor control
Frequency reported	About 60 to 80% of patients	Pathologic sweat tests in 26 to 94% across studies
Effect on cooling	Symptoms driven by the rise in core temperature	Less evaporative cooling, so core temperature can rise faster
Reversibility	Usually reverses within 24 hours as temperature falls	Can persist and may track with disability severity

The two problems reinforce each other: reduced sweating limits cooling, which makes the temperature rises that trigger Uhthoff phenomenon more likely.

How the two problems interact

Sweating is the body's main tool for shedding heat through evaporation. When MS reduces sweating, the body loses part of that cooling capacity, so core temperature climbs faster during exercise or in a warm environment. Because heat-sensitive MS symptoms are triggered by rising core temperature, impaired sweating can make Uhthoff phenomenon easier to provoke. This is why thermoregulation in MS is best understood as a two-part problem rather than heat sensitivity alone.

Researchers have studied practical countermeasures aimed at limiting the rise in core temperature, such as cooling before and during activity and staying in cooler environments. These are described in the literature as ways to preserve function during heat stress. This page describes that research at a general level and is not a set of instructions; decisions about managing MS and heat belong with the treating clinician.

What this means in plain terms

Heat sensitivity is one of the most consistent features of MS, affecting most patients, and it is a temporary, reversible response to a rise in core temperature rather than a sign of new damage. Separately, MS can reduce the ability to sweat, which lessens the body's cooling and can make heat sensitivity more pronounced. Neither feature is unique to MS, and neither on its own diagnoses it: MS is diagnosed by a clinician using history, examination, MRI, and other tests.

For someone already diagnosed with MS, understanding these two mechanisms explains why warm conditions, exercise, and fever can temporarily worsen symptoms. This page summarizes verified figures from published sources; it is educational and is not a diagnosis or medical advice.

Methodology and limitations

This page draws on Christogianni et al. 2018 (Temperature 5:208-223), the Davis, Jay & Wilson 2018 review of thermoregulatory dysfunction in MS, the Pintér et al. 2015 review of autonomic dysregulation in MS (Int J Mol Sci 16:16920-16952), the Uhthoff phenomenon StatPearls chapter, and the Walton et al. 2020 Atlas of MS third-edition prevalence paper (Mult Scler). Each figure (60 to 80% heat sensitivity, ~0.5 °C trigger threshold, reversal within 24 hours, 26 to 94% pathologic sweat tests, and ~2.8 million people with MS worldwide) was traced to those sources.

Limitations: the 60 to 80% heat-sensitivity range and the 26 to 94% sudomotor range are pooled figures that vary with how symptoms and abnormalities are defined and measured; individual studies fall at different points in those ranges. The 0.5 °C threshold is an approximate lower bound, and sensitivity varies between people. Prevalence of MS varies widely by region, and the worldwide figure is an estimate. Nothing here is a diagnosis or medical advice; anyone with new or worsening neurological symptoms should see a clinician.

Frequently asked questions

Why do people with MS feel worse in the heat?: Heat slows or blocks conduction in demyelinated nerve fibers, so signals that get through at a normal temperature stop getting through when the person warms up. This temporary worsening, called Uhthoff phenomenon, affects an estimated 60 to 80% of people with MS and usually reverses within 24 hours as body temperature falls.
How small a temperature change matters?: A rise in core body temperature of as little as about 0.5 °C can be enough to trigger temporary symptoms in heat-sensitive MS. Triggers include hot weather, hot baths, fever, and exercise.
Does MS reduce sweating?: It can. MS lesions can disrupt the pathways that control sweating, leading to reduced or regional loss of sweating and, in advanced disease, more widespread loss. Across studies, pathologic sweating-test results have been reported in 26 to 94% of MS patients.
Is heat sensitivity in MS dangerous or permanent?: Uhthoff phenomenon is temporary and reverses when body temperature returns to baseline; it reflects reversible conduction changes, not new damage. Reduced sweating is a separate issue that can limit cooling and may relate to disability severity. Any concerns should be discussed with the treating clinician.
Can sweating problems and heat sensitivity make each other worse?: Yes. Sweating is the body's main way of shedding heat by evaporation. If MS reduces sweating, core temperature rises faster, which makes the temperature-triggered worsening of Uhthoff phenomenon more likely.
Is this page medical advice?: No. It is a summary of verified figures from published sources. Only a clinician can diagnose MS or determine whether temperature-related symptoms relate to it.

Sources

Primary peer-reviewed studies and official sources first, then reviews and institutional framing (secondary).

Christogianni A, Bibb R, Davis SL, et al. Temperature sensitivity in multiple sclerosis: an overview of its impact on sensory and cognitive symptoms. Temperature (Austin). 2018;5(3):208-223. Uhthoff phenomenon in 60 to 80% of MS patients; core-temperature rises as small as ~0.5 °C can trigger symptoms. Full text
Davis SL, Jay O, Wilson TE. Thermoregulatory dysfunction in multiple sclerosis. Handb Clin Neurol. 2018;157:701-714. Reviews heat intolerance, central temperature control, and reduced sweating responses in MS. PubMed
Pintér A, Cseh D, Sárközi A, Illigens BM, Siepmann T. Autonomic dysregulation in multiple sclerosis. Int J Mol Sci. 2015;16(8):16920-16952. Pathologic sweating tests reported in 26 to 94% of MS patients; sudomotor dysfunction correlates with disability severity. Full text
Panginikkod S, Rayi A, Rocha Cabrero F, Rukmangadachar LA. Uhthoff phenomenon. StatPearls [Internet]. NCBI Bookshelf. Standard clinical reference: temporary, stereotyped, less-than-24-hour worsening of MS symptoms with raised core temperature. (secondary) StatPearls
Walton C, King R, Rechtman L, et al. Rising prevalence of multiple sclerosis worldwide: insights from the Atlas of MS, third edition. Mult Scler. 2020;26(14):1816-1821. Estimated 2.8 million people living with MS worldwide. (secondary) PubMed

How to cite this page

Sweat Explained. Multiple Sclerosis, Heat Sensitivity, and Sweating. Published 2026-07-20; last reviewed 2026-07-20. Available at: https://sweatexplained.com/research/multiple-sclerosis-and-thermoregulation

Please cite the original studies for the underlying figures. Journalists are welcome to link to this page; the charts are original renderings of the cited data.