Coronavirus FAQ: Why do so many folks say to drink lots of water before your vaccine?

Each week, we answer frequently asked questions about life during the coronavirus crisis. If you have a question you'd like us to consider for a future post, email us at goatsandsoda@npr.org with the subject line: "Weekly Coronavirus Questions." See an archive of our FAQs here.

People around the world have been rolling up their sleeve for a COVID shot for nearly a year – and millions more will be getting first doses and boosters in the weeks and months ahead.

Everyone wants to know: Will it hurt? And how can I make the hurt go away?

"I do think that people should know that many who get the vaccine will experience local side effects such as pain and redness at the injection site, muscle aches and tiredness," says Dr. Aaron Glatt, chief of infectious diseases at Mount Sinai South Nassau in New York. "Most get minimal to mild to more systemic symptoms over 24 to 48 hours. Expect that," says Glatt.

Here are some of the specific questions people are asking.

I got an email from the pharmacy where I scheduled my shot: "Drink at least 16 ounces of water 1 hour before your appointment to help prevent side effects." I've also heard people say that maybe that's a way to make the vaccine more effective. Is there any truth to that?

First of all, there is no evidence that a tall glass of water will make the vaccine more effective. "Staying hydrated in general is good for your health and organ function, but it does not have any effect on the immune system's response to the vaccine," says Dr. Suellen Hopfer, an assistant professor of health, society and behavior with the University of California/Irvine Program in Public Health.

Other experts agree about the effectiveness and don't think there's a direct impact on side effects.

"Drinking a glass of water won't make a difference [in terms of side effects or effectiveness]," says Dr. Amesh Adalja, senior scholar at the Center for Health Security at Johns Hopkins.

On the other hand, you shouldn't go into your appointment in a state of dehydration. "Being dehydrated increases your risk of feeling ill or fainting just as you might be dealing with vaccine side effects. "Don't take a vaccine or booster after running a marathon and becoming dehydrated," says Adalja.

It's good to always be well-hydrated, Adalja says. "Drink the normal amount you would drink." Benefits of hydration include regulating body temperature and delivering nutrients to cells.

Meanwhile, hydration might be helpful after the shot. If you do run a post-vaccine fever, a known possible side effect, the CDC recommends drinking plenty of fluids since during a fever the body uses its fluids to help cool down.

Does it make a difference which arm gets the shot – dominant or non-dominant?

"Sometimes the nurse or health-care worker administering the vaccine will ask which arm you use more or write with, in case it's sore afterward," says Dr. Suellen Hopfer. "But in the big picture, which arm you get your vaccine in will not matter as far as efficacy of the vaccine."

But choosing which arm gets the needle poke could get you back to work or video games sooner. "We recommend using the non-dominant arm for vaccine for the practical reason that if you get muscle aches at the injection site it won't be the arm you use the most," says Dr. Luis Ostrosky, chief of infectious diseases at the UTHealth Houston and a spokesman for the Infectious Diseases Society of America.

What about pain killers before and/or after the shot?

Dr. Ostrosky says the vaccine guidelines don't recommend pre-medicating with a pain reliever. Over-the-counter products can cause stomach irritation and bleeding in some people and should only be used when necessary. The guidelines from CDC do suggest over-the-counter pain relievers such as acetaminophen (Tylenol) or ibuprofen (Advil, Motrin) if you experience mild side effects such as pain at the injection site. "And if you do, by all means take something so long as you know it is safe for you, and follow the dosing instructions on the container," says Ostrosky. "Don't tough it out if you have pain, fever or malaise after the vaccine."

One more tip: The CDC recommends applying a clean, wet, cool cloth to the injection site to help relieve any pain you have and using or exercising that arm. Keeping your arm moving increases blood flow to the injection site, which can provide temporary pain relief, according to experts from Kaiser Permanente.

They also suggest thinking about what arm you sleep on and getting the vaccine in other arm if you think the pain might keep you awake.

Does smoking two "sticks" after harm vaccine efficacy?

That's what a reader wanted to know, so it is our responsibility to give you the dope on this burning question.

"We don't recommend use of marijuana following vaccination," says Dr. Ostrosky – that's because there's no proven benefit to doing so. "But there's unlikely to be any interaction with the vaccine if you do."

I feel silly, but I was nervous about getting the vaccine and now I'm nervous about getting the booster.

I've been counseling a lot of people about getting the vaccine in the hospital and at our clinics and know there is a lot of misinformation about the vaccines and a lot of scary things that are not true, and people are overwhelmed and become paralyzed, says Dr. Ostrosky. "What I want people to know is that the vaccines have been given to billions, they are safe, effective and like any other vaccine you have taken in your life. It's okay to be nervous about doing something new, but don't let it take over your life or send you out the door before you get your shot."

Dr. Ostrosky says that for people who have an anxiety disorder, they may need to take their medication close to the time they get their vaccine and may even need to work with their therapist before going to get the shot. "That is totally appropriate," Ostrosky says. "Go with a friend to get the shot if that will make it easier for you," advises Dr. Adalja.

Fran Kritz is a health policy reporter based in Washington, D.C., who has contributed to The Washington Post and Kaiser Health News. Find her on Twitter: @fkritz

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