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Your Guide to Determining If You Should Exercise When Sick or How Long to Wait

It's cold/flu/COVID-19 season. Here’s how to navigate your workouts if you get ill.

running with a fever, running when sick
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A breadth of research links regular exercise with better immune function, including one scientific review published in 2019 that says exercise has an anti-inflammatory effect and can improve defense activity. But what about if you’re in the midst of illness?

If exercising when you’re well offers so many benefits to your immune response and lowers inflammation, it would make sense that at least a small burst of activity when you’re sick could offer a boost, right?

Not so fast. Unfortunately, it doesn’t tend to work that way, according to Vivek Cherian, M.D., a Chicago-based internal medicine physician.

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“In most cases of illness, it’s better not to exert your body with a workout,” he tells Runner's World. “That’s because giving yourself adequate rest time can shorten your recovery window.” Exercising too soon might drag out your illness and keep you sidelined for longer than you would be otherwise.

There are some exceptions to this guideline, so here’s a look at what to keep in mind if you’re sniffling, have a fever, or you’re COVID-19-positive, but still feeling restless to lace up and get moving.

If you have a cold…

After symptoms subside, it’s best to wait 2-3 days to return to exercise.

Even though colds are caused by viruses just like the flu and COVID-19, symptoms tend to be much milder, according to the National Institutes of Health. That includes sore throat, congestion, and runny nose. If these feel minor, almost like allergies, then you’re likely safe to go on a run, but even then, you want to dial the intensity way back, suggests trainer Mike Matthews, C.P.T., author of The Little Black Book of Workout Motivation.

“Sometimes, you might not even have a cold, it might just be a bad night of sleep, and all you’ve got is a scratchy throat and you’re a little tired," he tells Runner's World. “The trick is to stay aware so that you see how you feel as the workout progresses. Start with an easy amount of exercise, like going for a walk, and take note if you begin to feel worse. I've made the mistake of ignoring that and paid for it later with more severe symptoms.”

The toughest part, he adds, is that it’s best to wait two or even three days after cold symptoms subside before getting back to your usual run. If you’re on a run streak or you simply hate the thought of “losing” a day, Matthews suggests you keep your training sessions easier and shorter, similar to de-loading in a strength training routine.

“For a lot of us, the challenge is having the discipline to rest for longer than we think we need,” he says. “But taking your foot off the gas is the better way to go.”

If you have the flu…

After symptoms subside, it’s best to wait 7-10 days to return to exercise.

A little congestion and mild fatigue from a cold make the decision about whether to run tougher, but when it comes to the flu, there’s usually less doubt, says Cherian. That illness often socks you with fever, chills, shortness of breath, and exhaustion.

“Absolutely refrain from exercising when you have these symptoms,” he says. In fact, running with a fever is never smart, as it can raise your internal temperature. Cherian adds that it’s best to wait seven to 10 days before easing back into an exercise routine. Although the timeframe will be different for everyone, he says trying to bounce back faster could cause a relapse and plunk you down right where you started.

That said, some symptoms may not be resolved even after 10 days, and in that case, the decision about whether to run will usually be made based on what those symptoms are.

“If you have a runny nose but otherwise feel fine, it’s okay to resume exercising,” says Cherian. “If you still have some shortness of breath or congestion, I’d recommend hold off. Bottom line, if you’re unsure, run it by your primary care doctor prior to resuming your exercise routine.”

If you have COVID-19…

After symptoms subside, it’s best to wait at least 10 days to return to exercise

As we’ve all heard umpteenth times, COVID-19 can hit people at dramatically different levels, with some having zero symptoms and others heading to the hospital. There’s plenty of reasons to skip working out while you have the virus—not just because of the discomfort with symptoms, but also due to potential harm post-recovery—but you also want to be cautious as you’re returning to exercise post-illness. (Research shows you should be at increased risk of injury.)

Even when you seem to be on the other side, that doesn’t always mean you’re safe to ramp up on the intensity, according to Matt Fitzgerald, author of Run Like a Pro (Even If You're Slow).

“I got infected early in the pandemic and though I seemed to recover, I subsequently developed long COVID-19 and have been unable to run for a year,” he tells Runner's World. “I'll never know for sure if returning to training too quickly contributed to my current condition, but as a coach, I now take a very conservative approach with my athletes.”

That includes adhering to published guidelines for those who develop symptoms and/or test positive. Even in the mildest cases, that means 10 days off, followed by a full clinical assessment, and a gradual return to training with close monitoring.

“Take the long view on this one, because the old rules don’t apply with Covid,” he says. “Do not take this virus lightly, recovery is different for this than with other viruses.”

How to ease back into exercise post-sickness

Whether you’re dealing with a cold, flu, or COVID-19—or maybe even two of those simultaneously, because that is possible, Cherian says—obviously you want to take it slow as you get back to your healthy self.

That might mean walking more than you run, or even doing some lower-impact cross-training activity, like yoga or swimming. Another major consideration is to stay aware, even when you feel well.

“Your symptoms could flare up again, and early signs might be increased heart rate or just feeling tired,” says Cherian. “Right now, it’s a good idea to incorporate more rest into your routine even if you’re healthy, considering we’re at the height of the sick season.”

No matter what your illness, heart rate is a smart metric to keep in mind as you ease back into activity, according to Robert Greenfield, M.D., cofounder of California Heart Associates. He tells Runner’s World that your heart rate should go back to normal within a few minutes of your cooldown, but that if it’s still elevated for 10 to 15 minutes, that’s a concern and you should consider getting checked out. That means you’d need to know what’s “normal” for you, which can change as you get fitter since your cardiovascular system is more efficient, he says. Either way, if you feel like your pulse is racing when you’re running and especially if you feel lightheaded, those are signs to cut your run short.

Other signs that you should probably slow to a walk are intense fatigue, joint pain, feeling like you can’t catch your breath, nausea, and chills. If your symptoms are severe, such as chest pain, shortness of breath, or you feel like your heart is skipping beats, Greenfield says your next stop should be the emergency room.

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