Some patients are preemptively taking over-the-counter pain medications before receiving a Covid-19 vaccine to stave off potential side effects, such as muscle aches or pains—but researchers warn that common painkillers may reduce the immune system's response to some vaccines.

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Could preemptive painkiller doses interfere with the Covid-19 vaccine?

According to ABC News, the side effects of currently available Covid-19 vaccines, which are generally mild, are spurred by the way the vaccines activate the body's defense system to build immunity to the new coronavirus.

In other words, these side effects are evidence the vaccine is doing exactly what it should be doing—and that's why some experts are cautious about preemptively taking painkillers, ABC News reports.

Specifically, experts are concerned that taking painkillers may slow the immune system's response to the vaccine, reducing the vaccine's effectiveness.

For instance, a study published in the Journal of Virology found that nonsteroidal anti-inflammatory drugs, such as ibuprofen, could hinder the production of antibodies and other elements of the immune system response to the new coronavirus. And a 2016 study from Duke University found children who received OTC painkillers before receiving their childhood vaccinations had fewer antibodies than those who hadn't taken painkillers—although their antibody levels remained sufficient to provide protection.

Colleen Kelley, an associate professor of medicine at Emory University School of Medicine, theorized the altered immune system response could be due to the painkillers reducing inflammation caused by the immune system.

"The immune system generates a response through controlled inflammation. (Pain relievers) can reduce the production of inflammatory mediators," she said. "So, this is the potential mechanism for a reduced immune response to vaccination if you take these medications."

That said, a 2020 meta-analysis found that children who took OTC painkillers before receiving vaccines did not see a significant reduction in effectiveness in the vaccines they received.

In light of this uncertain and conflicting evidence, many public health experts, as well as CDC and the World Health Organization (WHO), presently recommend against taking OTC painkillers before receiving a Covid-19 vaccine.

"You always would like an optimal response to your vaccine," William Schaffner, an infectious disease specialist and professor of preventive medicine at Vanderbilt University Medical Center, said. "We are recommending that unless people have a substantial reaction to the first dose that they hold their [painkillers]."

Schaffner added one caveat: If you've been taking OTC painkillers for another condition, you should continue taking them, as stopping them could cause more harm than good.

Is it OK to take a painkiller after receiving the vaccine?

As for taking painkillers after a Covid-19 vaccination, CDC recommends patients first monitor for any side effects—but if any do present, both CDC and WHO said it should be fine for people to use OTC painkillers.

"If fever, chills, headaches develop after injection," people can use OTC painkillers, Wildes said. But she added that people should be sure to report any serious or significant side effects to their providers.

Kelley added, "It's perfectly fine to take NSAIDs or Tylenol if you are feeling unwell after vaccination" (Llewellyn, ABC News, 1/27; Fritz, Washington Post, 2/3; Rodriguez, USA Today, 1/29; Anderson, Becker's Hospital Review, 2/3).