Like with any vaccine, there is a risk of developing side effects with the flu shot. Most side effects are mild and at the injection site. Stomach upset is not very common, but is possible.

Every year, health experts recommend everyone who’s able to get an annual influenza vaccine. Most people get this vaccine via a flu shot in the arm.

The flu shot is designed to help protect against the four most anticipated strains of the virus to be in circulation.

Any side effects from these vaccines are caused by your immune system activating, which starts building protection in your body against influenza. Every body is different, so not everyone will have side effects, and when they do, side effects can vary.

Here’s what you need to know about the likelihood of an upset stomach after getting an annual flu shot, other possible side effects, and what you can do about them.

Stomach pain is a possible side effect of a flu shot, though how often it happens is unclear.

Like other side effects, stomach pain or upset should resolve within a few days.

According to a 2019 study of vaccinated children, about 5 out of 210 (fewer than 1%) experienced abdominal pain after getting a flu shot. A stomachache is also a side effect associated with nasal spray versions of the flu vaccine.

Diarrhea is another possible side effect of the flu shot. It may last 1–2 days.

According to the same 2019 study of vaccinated children, the same rate of diarrhea was noted as abdominal pain (about 5 in 210 participants).

But while the authors note that the vaccine likely decreased the rate of such symptoms, more research is needed to determine how many people experience diarrhea as a side effect of the flu shot itself.

According to the American Heart Association, you may experience nausea as a side effect of the flu shot. This side effect is usually mild and gets better on its own within a few days.

Other GI side effects from a flu shot are possible, but they’re still not common.

A 2018 study of older adults reported that rare GI side effects seen after vaccination included:

However, the researchers note that it’s not clear whether such GI side effects are linked to the flu shot or if they developed for other reasons. These GI side effects were also not very common. More targeted research is needed to determine cause and effect.

Aside from an upset stomach and GI side effects, there are other short-term and long-term side effects of the flu shot:

Short-term side effects of flu vaccine

The flu shot’s short-term side effects include:

  • injection site reactions, such as redness, soreness, or swelling
  • fatigue
  • fainting
  • fever
  • body aches
  • headache

Like other flu vaccine side effects, these are usually mild and will resolve after a few days without medical intervention.

Long-term side effects of flu vaccine

While rare, severe and life threatening allergic reactions are possible after receiving the flu vaccine. These may be attributed to allergies to some of the vaccine ingredients, such as gelatin or egg protein.

Another possible but rare complication is developing Guillain-Barré syndrome. However, this only occurs in about 1 or 2 vaccinations per 1 million. This is lower than the chances of having severe complications of the flu itself.

The side effects of the flu vaccine are similar in babies as in other age groups. Babies may also experience a decreased appetite, tiredness, and fussiness after getting the flu vaccine.

You can help ease an upset stomach by:

  • drinking plenty of clear fluids, such as water or electrolyte beverages
  • drinking herbal teas such as ginger
  • resting
  • following a temporary BRAT diet (bananas, rice, applesauce, and toast) for up to 48 hours
Medical emergency

Get medical help right away if you’re experiencing possible symptoms of an allergic reaction to the flu shot. Symptoms of a severe reaction may develop within minutes or a few hours after getting the vaccine and may include:

  • wheezing
  • difficulty breathing
  • hives
  • dizziness
  • increased heart rate
  • pale skin
  • weakness

You may also want to reach out to a doctor if you’re having flu shot side effects that last longer than a few days.

You should be able to resume your usual activities after getting a flu shot. If you do experience side effects, you may want to try some of the following:

  • Take over-the-counter pain relievers for arm pain, fever, or muscle aches. These include acetaminophen (Tylenol) or ibuprofen (Advil) 2 hours before your scheduled flu shot. However, aspirin is not recommended for people under age 18.
  • Get plenty of rest.
  • Drink plenty of water to help prevent dehydration.
  • Stay warm with blankets or by wearing layers.

The Centers for Disease Control and Prevention (CDC) recommends the flu vaccine for everyone 6 months of age and older. Getting an annual flu shot helps decrease your risk of developing flu, as well as complications if you do get sick.

This is also particularly important if you have a higher risk of developing more serious complications, such as if you’re older than age 65, younger than age 2, or pregnant.

You also have a higher risk if you have any of the following conditions:

  • chronic lung disease, such as asthma or COPD
  • kidney disease
  • heart disease
  • a history of stroke
  • cancer
  • HIV
  • metabolic disorders

Who should avoid it

On the flip side, the CDC does not recommend the flu shot for the following groups:

  • infants under 6 months of age
  • people with severe allergies to gelatin or other ingredients in a flu shot
  • anyone with a history of a severe reaction to a flu vaccine
  • people who are currently ill
  • those with a history of Guillain-Barré syndrome (you can ask a doctor first)

A stomachache and other GI ailments are uncommon but possible side effects from the flu shot. Like other flu vaccine side effects, they are usually temporary and should go away on their own.

Speak with a doctor if any stomach upset or other side effects last longer than a few days.