Why Your COVID Booster Shot Makes You So Thirsty

Feeling dehydrated after rolling up your sleeve for a vaccine? Here's what's going on.
Some people report feeling very thirsty after their COVID shot or booster, but it's not an officially recognized side effect.
laflor via Getty Images
Some people report feeling very thirsty after their COVID shot or booster, but it's not an officially recognized side effect.

Spending the hours after you get your COVID-19 booster dose feeling like you can’t gulp down enough water? It’s not just you: The internet is full of anecdotes from people saying their boosters — and their initial COVID doses, in some cases — left them feeling parched.

So what gives? Here’s what you need to know.

First, thirst is NOT an officially recognized side effect of COVID vaccines.

The most common COVID vaccine side effects are pain, redness or swelling where you got the shot, as well as possible tiredness, headache, muscle pain, chills fever and nausea, according to the Centers for Disease Control and Prevention. That includes booster doses, too.

It’s worth noting that many experts are reluctant to even call those mild to moderate responses “side effects.” Really, they’re just “expected symptoms when our immune system is ramping up and doing its job,” explained Natasha Bhuyan, a physician at primary care practice One Medical. (That’s in contrast to serious but rare side effects that can occur.)

Bhuyan said she hasn’t had any patients who have experienced thirst after their shots. That doesn’t mean it’s not happening, however. Millions of people have gotten vaccinated against COVID-19 over the past year, and there have been scores of anecdotes about side effects not yet confirmed, or even explored in studies — some much more credible than others. That includes everything from food cravings to menstrual changes. And yes, thirst.

“People have different symptoms when they get their immunizations, but I have heard of people feeling a little bit thirsty. That’s not an uncommon finding,” said LaTasha Perkins, a family physician in Washington, D.C.

There are a few theories as to why some people feel parched after their booster.

There are several reasons why someone might feel thirsty after getting boosted. One is simply nerves. “If someone is nervous about the vaccine, that can cause dry mouth, as anxiety is linked to dry mouth,” Bhuyan said.

Then there’s the really obvious possibility, Bhuyan said: that people who were already dehydrated might feel particularly parched after getting a shot. They might just notice it more because they’re paying closer attention to how they’re feeling in general.

But other physiologic explanations are also possible. For one, developing a fever could contribute to dehydration. That’s not specific to COVID vaccines; that risk is always there as your body loses salt and fluids.

You might also feel thirsty simply because your immune system is working hard.

“Every human is about 70% water, and a lot of that is in the vessels in your blood,” Perkins explained. When your body mounts an immune response, it asks a lot of your tonsils, adenoids and white blood cells.

“When you get the booster, it can definitely make your lymph nodes and your adenoids activate, which could make you feel a little bit thirsty,” Perkins said. “But it doesn’t mean anything’s wrong.”

Pre- and post-booster self-care is important.

If you haven’t already gotten boosted, do what you can to take care of yourself beforehand. Eat a nutritious meal. (If you’re looking for specifics, here are some good options.) To the extent it’s possible, avoid over-the-counter medicines like ibuprofen and aspirin beforehand, because it’s not clear whether they could impact how the vaccines work.

Some people have wondered whether drinking a big glass of water before you roll up your sleeve will help prevent side effects. Experts say it won’t, although hydration is always important. (You can tell if you’re in the general ballpark if you’re peeing every hour or two, or your urine is relatively clear, Perkins said.)

Bhuyan said she advises patients to be well-hydrated before they go in for a vaccine dose, because being parched could exacerbate possible symptoms after, like headaches.

Then steel yourself for the fact that you might feel not-so-great after your booster dose as your immune system does its thing. The CDC says reactions to boosters have been basically the same as to the initial doses of either of the two mRNA vaccines or the Johnson & Johnson shot. But alas, there’s no accurate way to say whether your response will be better or worse than it was the first time around.

“Some people have zero symptoms other than their arm being sore. Others feel sluggish or tired, or they mount a fever,” Perkins said. “But keep in mind: It’s only your immune system responding.”

Experts are still learning about COVID-19. The information in this story is what was known or available as of publication, but guidance can change as scientists discover more about the virus. Please check the Centers for Disease Control and Prevention for the most updated recommendations.

Go To Homepage