Runner's Flu: Why You Feel Sick After a Race and What to Do

Runner's flu is a name runners use. It describes feeling achy, run-down or cold-like. This happens in the days after a marathon or another long and hard race. Sometimes the cause is your body recovering from the effort. Sometimes it's a real cold you picked up around race weekend. The first steps stay the same in either case: eat, drink, rest and sleep, and don't run while you have a fever.

This guide helps you tell the two apart, spot the symptoms that need a doctor and decide when to run again. It draws on studies of marathon and ultramarathon runners, along with current CDC and Mayo Clinic guidance.

What runner's flu actually is

"Runner's flu" is a nickname, not a diagnosis, and it doesn't mean you have influenza. The feeling usually comes from one of two things, and sometimes both at once.

The first is your body's recovery response. A marathon causes microscopic muscle damage and sets off a large inflammatory reaction. In one small study of Copenhagen Marathon finishers, the inflammatory messenger IL-6 was 128 times higher right after the race than before it. Sore legs, heavy fatigue, poor sleep and a general "off" feeling can follow for a few days without any germ involved.

The second is a real infection, usually a cold, which researchers call an upper respiratory tract infection. Crowded start corrals, travel, short sleep and a big physical effort all land in the same weekend. Still, cold-like symptoms don't always mean a germ. One study swabbed athletes every time they reported cold symptoms over five months, and researchers found a germ in fewer than 3 in 10 episodes, most often rhinovirus, a common cold virus. So a runny nose after a race isn't proof that you caught something.

Recovery response or a cold? How to spot the difference

You can make good choices without an exact label. Use the row that best matches what you're noticing.

The fever line comes from the CDC, which defines a fever as a measured temperature of 100.4°F (38°C) or greater. The split between symptoms above the neck (nose and throat) and below it (chest, stomach or whole body) follows Mayo Clinic's guide to exercise and illness.

When symptoms need a doctor

Most colds and post-race slumps settle on their own. Some symptoms point to something more serious, though. Don't try to wait these out:

• Dark urine plus muscle pain, cramps or weakness. The CDC lists abnormally dark (tea or cola-colored) urine among the signs of rhabdomyolysis and advises immediate care.
• Confusion, repeated vomiting or a severe headache after the race. These can be signs of low blood sodium (hyponatremia). MedlinePlus lists confusion, headache, nausea and vomiting among its symptoms. At the 2002 Boston Marathon, 13% of tested finishers had low sodium. It was linked to weight gain during the race (a sign of drinking more than you sweat out) and to finish times over four hours. Don't try to treat these symptoms by drinking more plain water. Get medical help.
• Trouble breathing, fast breathing or chest pain. MedlinePlus advises calling 911 for sudden, crushing pressure in your chest, for chest pain that spreads to your jaw, left arm or between your shoulder blades, and for sudden, sharp chest pain with shortness of breath.
• Signs of dehydration that don't improve as you drink.
• A fever that lasts more than 4 days.
• Symptoms that last more than 10 days without getting better, or that improve and then come back worse.
• A chronic condition, such as asthma, that gets worse.

Apart from chest pain, the last five points come from the CDC's guidance on colds, which notes that its list isn't complete. If something feels severe or wrong, get it checked.

What to do during the first few days

A marathon calls for recovery time. That applies in the days afterward even if you don't feel sick. The basics are the same if cold symptoms show up.

1. Rest from training. Walking around the house is fine and so is a short and easy stroll. Workouts can wait.
2. Sleep more than usual if you can. Naps count.
3. Eat regular meals that have carbohydrates and protein, even if your appetite is low.
4. Drink steadily throughout the day. Warm drinks can soothe a sore throat.
5. Test if you suspect flu or COVID. The CDC suggests testing, especially if you're at higher risk of severe illness. Contact a doctor early if that applies to you.
6. Keep your distance from others for as long as you have a fever. Protect your family and training partners by washing your hands and covering coughs.

Be cautious with pain relievers. Follow the label, and ask a pharmacist or doctor first if you have kidney problems or take other medicines. Think twice about taking ibuprofen during long races, too. In a trial during 50-mile ultramarathons, runners given ibuprofen developed acute kidney injury more often than those given a placebo (52% vs 34%). The trial was small and the difference wasn't statistically conclusive, but the authors advise caution.

How long runner's flu lasts

Most post-race soreness and most colds ease within a week. A typical course goes like this:

Symptoms don't always show up right away with a cold. Studies of post-race illness counted infections for one to three weeks after the race. A cold that starts midweek may still trace back to race weekend.

When you can run again

These rules can help with your decision about when to run again.

• Fever means no running. Wait until you've gone at least 24 hours without a fever (without taking a fever reducer) and your symptoms are improving overall. That matches the CDC's rule for returning to normal activities.
• Symptoms below the neck mean no running. That covers chest congestion, a hacking cough, an upset stomach, widespread aches and heavy fatigue.
• Mild symptoms above the neck can allow easy exercise if you feel up to it, such as a stuffy nose with no fever. Keep it shorter and easier than normal. In a lab study, young adults given a mild cold virus who exercised moderately for 40 minutes every other day had no worse or longer symptoms than those who rested.
• Ease back in after a fever. UW Medicine sports physician Mark Harrast notes that significant exercise after a fever may raise the risk of exertional heat stroke.

For example, a first run back might be 20 to 30 minutes at a relaxed, conversational pace. If you feel fine that day and the next morning, add time over the following runs. Return to speed work or long runs only after you've built that time back up. Give yourself more time if your heart rate is higher than usual on easy runs.

You're still recovering from the race itself even if you never got sick. Save hard workouts for later and keep runs easy until your legs feel normal again. See why your legs feel heavy when you run if your legs stay heavy for weeks.

Why hard races can leave you feeling sick

Hard, long efforts change your immune system for a few hours. Stress hormones such as cortisol and adrenaline rise during a long run. A 2018 review describes immune measures that often drop 15% to 70% in the hours after heavy exertion. For years, this was called the "open window": a short period of weaker defenses when germs could slip in.

That theory is now debated. Another review argues that the drop in white blood cells after exercise mostly reflects cells moving out of the blood into tissues, where they keep watch, rather than true immune suppression. In a 2020 debate paper, exercise immunologists on both sides agreed on one point: getting sick around hard events has many causes. These include anxiety, poor sleep, travel, exposure to germs, gaps in nutrition and harsh weather.

The good news is that you can influence most of those factors.

How common post-race illness is among runners

The numbers are real. They're smaller than what you might have seen reported in other places. Most come from runners reporting their own symptoms.

Taken together, the studies show a modest rise after marathons and ultras but not after shorter races, and the risk isn't the same for everyone. The Stockholm data also carry a clear warning: starting a marathon while you're sick, or barely over it, is a bad bet.

How to reduce your odds of getting sick before the next race

There's no plan that can prevent every illness. These habits cover the factors that researchers agree on:

• Protect your sleep in race week. In a study that gave healthy adults a cold virus, people who slept less than 6 hours a night were about four times as likely to catch a cold as those who slept more than 7. The volunteers weren't runners, but race week is when sleep tends to slip.
• Wash your hands often, especially at the expo, before eating and after the finish, and keep your hands away from your face. The CDC says hand washing cuts respiratory illnesses like colds by 16% to 21%.
• Don't race a marathon while you're sick. Step down to a shorter distance or defer if you're still getting over an illness.
• Eat enough for your training, and fuel the race. Low energy availability, or not eating enough for the training you do, is one of the risk factors for illness in athletes, along with long-haul travel and high stress. Get enough carbohydrates to keep your glycogen stores, the carbohydrate stored in your muscles and liver, topped up between hard sessions. Practice your race fueling in training, and see how to avoid hitting the wall for a plan.
• Limit close contact when you can. Spend less time in crowded indoor spaces around race day, carry hand sanitizer, and wear a mask in crowds if you like. UW Medicine's prevention list also includes staying current on vaccinations.
• Keep your training load realistic, and build mileage gradually. In the Los Angeles Marathon study, runners logging about 60 miles a week or more had twice the odds of reporting an illness in the two months before the race as those running under 20.

Do vitamin C or other supplements help?

Vitamin C has a finding specific to people under heavy physical stress, and the details matter. A Cochrane review found regular vitamin C didn't reduce colds in the general population. But in five small trials with marathon runners, skiers and soldiers under heavy physical stress, it cut the risk of a cold roughly in half. Those five trials included fewer than 600 people. Starting vitamin C after symptoms begin showed no consistent benefit.

The evidence on other supplements is mixed. When you're actually short on them, vitamins help most. Talk to your doctor before adding supplements (especially if you take other medicines).

The bigger wins are less exciting: sleep, food, clean hands and an honest call about whether you're well enough to race. If you do get sick anyway, rest until you've been fever-free for 24 hours and are improving, then build back with easy runs. As Mayo Clinic puts it, a few days off when you're sick shouldn't affect your performance.

Frequently asked questions

Is runner's flu real?

Yes, though it isn't a medical diagnosis. Several studies found more cold-like illness after marathons and ultramarathons. In a Los Angeles Marathon study, 12.9% of finishers reported an illness the week after the race, compared with 2.2% of similar runners who didn't race. Some post-race symptoms are also just your body recovering from the effort.

How long does runner's flu last?

Muscle soreness usually peaks 2 to 3 days after a hard race. Colds last less than a week in most cases. See a doctor if a fever lasts more than 4 days, if symptoms last more than 10 days without improving, or if they improve and then get worse.

Why do I feel like I have the flu after running?

A very long or hard run causes muscle damage and a large inflammatory response. This can leave you sore, tired and feeling off for a few days. You may also have caught a cold around race time. A fever at 100.4°F (38°C) or higher points to an illness rather than normal recovery.

How do I get rid of runner's flu fast?

There isn't a quick fix. Rest from training. Eat regular meals with carbohydrates and protein and drink fluids. Get some sleep too. Starting vitamin C after symptoms begin hasn't shown a consistent benefit in trials. Test if you suspect flu or COVID. Call a doctor about any red-flag symptoms.

Can I run with runner's flu?

Not with a fever or symptoms below the neck, such as chest congestion, a hacking cough, an upset stomach or aches all over. An easy and shorter run is usually OK with mild symptoms above the neck if you feel up to it. After a fever, wait until you've gone 24 hours without one (without fever reducers) and are improving. Then ease back in with short and easy runs.

Can you get runner's flu after a half marathon or 10K?

You can catch a cold any time, but the research points mostly to marathons and ultras. A study of 5K to half-marathon runners found no rise in illness the week after the race. Shorter races can still leave you sore and tired for a day or two.

Is runner's flu contagious?

It's contagious if it's an infection like a cold or flu. Your body's recovery response to a hard race isn't contagious. You can't always tell which you have. Cover coughs and wash your hands often. When you have a fever, keep your distance from others.

Alex Gorovenko

I completed my first 10K on a dare. In a year, I ran a half-marathon. Another year later, I finished a marathon race. Today I run 4 marathons a year and a half-marathon every week. I learned everything about running the hard way. So, I help runners achieve better results easier.