Why does heat make MS worse?

Most people with MS notice it at some point: a hot day, a warm bath or a bout of flu, and suddenly symptoms that were manageable feel much heavier. Vision blurs, legs feel weak, fatigue lands hard. This is one of the best documented patterns in MS, and it has a name.

MS damages myelin, the insulating layer around nerve fibres. Damaged nerves still conduct signals, but with very little margin for error. A small rise in core body temperature, sometimes as little as a quarter of a degree, slows conduction along those nerves even further, and signals that were just about getting through start to fail.

The result is a temporary worsening of existing symptoms. It was first described in people whose vision blurred after exercise, and it is now known as Uhthoff's phenomenon.

Heat sensitivity is a pseudo-relapse. Symptoms flare while you are warm and settle once your temperature returns to normal, usually within minutes to hours. It is not new inflammation and it does not cause lasting nerve damage. That said, if symptoms are new, or persist for more than 24 hours after cooling down, contact your MS team, because that may be a genuine relapse or an infection.

• Hot, humid weather and direct sun
• Hot baths, showers, saunas and hot tubs
• Vigorous exercise, which raises core temperature
• Fever from a cold, flu or urinary tract infection
• Warm indoor spaces, heavy clothing and hot meals or drinks
• Hormonal changes, including hot flushes

• Pre-cool: a cool shower or a cold drink before activity can extend how long you last.
• Cooling products: cooling vests, neck wraps, wrist bands and cool packs on pulse points work well.
• Stay hydrated: cold water and ice lollies help regulate temperature.
• Time your day: shift errands and exercise to early morning or evening.
• Dress light: loose, breathable layers you can remove easily.
• Cool the room: fans, blinds closed during the day, windows open at night.
• Exercise smart: swimming in a cool pool is one of the best options, since the water carries heat away.

Not everyone reacts the same way. Some people find cold weather increases stiffness, spasticity or pain, and a smaller group feel better in heat. Knowing which direction affects you most is genuinely useful when planning holidays, work and daily routines.

Because heat effects are temporary and vary from person to person, the clearest way to understand yours is to record symptoms alongside the conditions of the day. Over a few weeks, a pattern usually emerges, and once you can see it you can plan around it.