If you’re sensitive to temperatures — especially heat — and your multiple sclerosis (MS) symptoms feel worse when you have a fever, you’re not alone. MS causes damage (called demyelination) to the myelin sheaths of nerve cells in the central nervous system. A higher body temperature makes it harder for axons of demyelinated nerves to send electrical signals. Even small increases in core body temperature can cause MS symptoms to get worse.

People experiencing inflammation related to MS tend to have warmer body temperatures than those without MS. Certain medications for MS may also cause infections (which may result in fevers) as a possible side effect. The definition of a fever is an internal body temperature at or above 100.4 degrees Fahrenheit (38 degrees Celsius). Because fever can make MS symptoms feel worse, it’s helpful to understand the underlying causes of fever, how to manage fever, and when to seek medical attention.

Can Fevers Make MS Symptoms Worse?

Heat sensitivity or fever can make old MS symptoms pop back up or existing symptoms feel worse.

Some symptoms that may worsen include balance issues, fatigue, vision impairment, and cognitive problems. About 60 percent to 80 percent of people with MS experience worsening neurologic symptoms and clinical signs (changes in the normal healthy state) with exposure to heat. Some MyMSTeam members also describe worsening muscle aches and pain, muscle weakness, and movement problems with heat sensitivity:

• “I had a very high temperature a year ago. I couldn’t move my left leg, and my left hand was really clumsy.”
• “I had two bladder infections where I ran a temp. Both times, the fever affected my MS. I couldn’t stand, kept collapsing.”
• “Having a fever causes me to be numb.”
• “Got a sinus infection and fever. My MS is OK, but when I get a fever, I can’t move so well.”
• “I’m confused whether my myalgia is due to the fever, or if the fever has worsened my usual MS symptoms.”

The worsening of MS symptoms with fever is usually temporary. Once the body cools and gets back to a normal temperature, symptoms should reduce to the level that’s normal for you.

Do Fevers Worsen MS Disease Activity?

Although fevers cause symptoms to feel worse, they do not cause more disease activity. MS symptoms can get worse from fever without clinical worsening — new or larger lesions, detectable demyelination, or measurable nerve damage. A symptom flare-up without worsening disease is sometimes called pseudoexacerbation or Uhthoff’s phenomenon. When symptoms do flare, it’s important to check for a fever because a minor infection could be the culprit. For example, urinary tract infections (UTIs) are a common type of infection known to trigger MS symptom flare-ups.

Can MS Cause Fevers?

MS lesions can occur in areas of the brain that are responsible for body temperature regulation and the body’s response to changes in temperature. This is why many people with MS experience sensitivity to heat, although MS itself doesn’t necessarily cause fevers. Some people with MS may have higher body temperatures more often than others.

MyMSTeam members have reported recurring mild fevers. “I seem to get random low-grade fevers that make my MS symptoms worse,” shared one member. “I also have that issue,” replied another. “I mean really low-grade, like 98.9 degrees Fahrenheit. I can tell because I feel wiped out and my vision is messed up. It’s so frustrating!”

Research has shown that people with relapsing-remitting MS (RRMS) tend to have a higher body temperature, possibly due to inflammation related to RRMS. Although not necessarily fever, elevated body temperature in people with RRMS has been linked to worsening general and physical fatigue.

Fevers and Disease-Modifying Therapies

Each disease-modifying therapy (DMT) has possible risks and side effects alongside its benefits for treating RRMS. Some DMTs may cause a fever or raise the risk of contracting an infection that could cause a fever. According to the National Multiple Sclerosis Society, DMTs that list fever as a common side effect are:

• Interferon beta-1a (sold under brand names including Avonex and Rebif)
• Pegylated interferon beta-1a (Plegridy)
• Interferon beta-1b (sold under brand names including Betaseron and Extavia)
• Alemtuzumab (Lemtrada)
• Ocrelizumab (Ocrevus)

Because DMTs modify aspects of the immune system to reduce MS attacks, infections are common side effects of many DMTs. An infection may lead to fever as the body tries to fight off the virus or bacteria. Below are DMTs that list infections as a common side effect, including the type of infection they may be associated with:

• Ofatumumab (Kesimpta) — Upper respiratory tract infection
• Cladribine (Mavenclad) — Respiratory infection
• Ponesimod (Ponvory) — Upper respiratory tract infection
• Ozanimod (Zeposia) — Upper respiratory tract infection and UTI
• Alemtuzumab (Lemtrada) — UTI, upper respiratory tract infection, herpes viral infection, and fungal infection
• Mitoxantrone (Novantrone) — Upper respiratory tract infection and UTI
• Ocrelizumab (Ocrevus) — Respiratory tract infection and herpes infection
• Natalizumab (Tysabri) — UTI, lower respiratory tract infection

MyMSTeam members have shared their experiences about fevers as a side effect of DMTs. One member said, “I started an injected biologic DMT yesterday, and wow, I’m definitely having those flu-like symptoms. Can’t get rid of this fever!” Another shared, “My interferon shots for MS used to cause fevers.”

If you are experiencing fever and wonder if it is a result of your medications, talk to your health care provider or neurologist. They can help you figure out what may be causing fevers and how best to manage them.

Tips for Managing Fevers With MS

Managing fever is important for your body overall and especially for reducing the effect of worsening MS symptoms. The main goal when you have a fever is to keep your core body temperature as close to normal — around 98.6 degrees Fahrenheit (37 degrees Celsius) as possible. Make sure you have a thermometer at home to monitor your temperature.

Depending on the severity of the fever, it can usually be managed at home. Talk to your health care provider about over-the-counter medications and other strategies to keep your temperature down if and when fever strikes.

If you notice fevers as a side effect of your DMT, ask your doctor how to prevent them. One MyMSTeam member wrote, “When I was on an injectable DMT that caused flu-like symptoms, my neurologist told me to take over-the-counter painkillers after the injection to ward off any flu-like symptoms and it worked.” Be sure to consult your doctor before adding any medications, even those available over the counter, to avoid dangerous drug interactions.

Tips for managing fevers at home include:

• Drink plenty of clear fluids.
• Keep your temperature consistent — avoid extreme changes in temperature, such as going from an air-conditioned room to hot weather outside.
• Rest as much as you need, especially because elevated body temperature can increase fatigue.
• If you experience chills with your fever, warm up gradually. Adding too much heat quickly can increase your temperature too much.
• If your temperature spikes, cool down gradually, such as with a cool cloth or a lukewarm shower.
• To help manage your temperature at night, keep blankets by your bed to add on or peel off as needed.

When To Seek Medical Attention

Although fever is a common response to mild illness or infection, it can also be a sign of other potentially serious conditions, such as respiratory infections or COVID-19. If you are experiencing fever with other COVID-like symptoms, consider taking an at-home COVID-19 test or scheduling a test at a local testing site or your doctor’s office.

The following symptoms may be a sign of more serious illness, and you should seek medical attention from a health care provider, urgent care, or emergency room as soon as possible.

• Fever above 104 degrees Fahrenheit (40 degrees Celsius)
• High fever for more than three days without breaking
• Fever with other signs of underlying potentially serious conditions — rash, severe headache, neck stiffness, confusion, difficulty staying awake, severe diarrhea, sensitivity to light

Talk With Others Who Understand

MyMSTeam is the social network for people with MS and their loved ones. On MyMSTeam, more than 191,000 members come together to ask questions, give advice, and share their stories with others who understand life with MS.

Do you experience frequent fevers or worse MS symptoms with fever? Have you found effective ways to manage fevers? Share your experience in the comments below, or start a conversation by posting on your Activities page.