Does the Flu Shot Cause Diarrhea? What the Package Inserts Actually List
Flu shots are on offer everywhere and the search bar is asking a question the official lists do not quite answer. So we read all eight package inserts on this year’s CDC list and pulled every gut row — and found that the scariest number online belongs to toddlers, and matches its comparison group exactly.
Diarrhea is not a listed side effect of the flu shot. CDC's side-effect lists name soreness, redness or swelling at the site, headache, fever, muscle aches, nausea and fatigue for the shot, and add vomiting for the nasal spray in children; the word diarrhea appears on none of them. In the FDA package inserts for the eight 2026-27 vaccines, five (Fluzone, Fluzone High-Dose, Flucelvax, Fluarix, FluLaval) did not solicit any gastrointestinal symptom in adult trials. Where adults were asked, rates matched the comparison: Flublok nausea 6 percent versus 5 percent on placebo in 4,503 adults aged 18 to 49; Fluad diarrhea 4.8 percent versus 4.5 percent against another flu vaccine in adults 65 and over. The widely shared "18 percent diarrhea" figure comes from Flucelvax's table for children aged 6 months to 3 years, where the comparison vaccine group reported 20.2 percent in the youngest infants, meaning the shot did not change the number. The nasal spray shows decreased appetite in 21 percent of children aged 2 to 6 versus 17 percent on placebo. An analysis of 31,983 older adults found no difference in serious gastrointestinal events between high-dose and standard vaccine (relative risk 0.91), and the Cochrane review concluded that rates of nausea and vomiting are uncertain because trials rarely measured them. No insert mentions stool colour. Educational, not medical advice.
Flu shots went on offer nationwide at CVS and MinuteClinic on August 27, and CDC’s clinical guidance for the 2026–27 season says vaccination “should ideally be offered during September or October.” Which means that right now, a lot of people are typing a question into search that the official side-effect lists do not quite answer: can the flu shot give you diarrhea? The autocomplete goes further — “flu shot poop,” “flu shot green poop,” “flu shot nausea a week later.”
The honest way to answer is to read the documents. Every flu vaccine licensed in the United States has an FDA package insert with a table of “solicited adverse reactions” — the symptoms trial participants were asked to log on a diary card for a week, side by side with whatever the trial compared against. We read all eight inserts on this year’s CDC list, plus CDC’s own side-effect pages, and pulled every gastrointestinal row. Here is what they say, what they do not, and why the scariest-looking number is not what it looks like.
What CDC listsNausea, yes. Diarrhea, no.
CDC’s key-facts page lists the common side effects of the flu shot as “soreness, redness, and/or swelling where the shot was given, headache, fever, muscle aches, nausea, fatigue.” For the nasal spray in children it lists “runny nose, wheezing, headache, vomiting, muscle aches, fever (low grade).” A second CDC page on vaccine safety drops nausea from the injectable list entirely. The nasal-spray page keeps vomiting. Across all three, the word diarrhea does not appear for either product. So the short version, from the agency’s own lists: nausea is a recognised but uncommon shot side effect, vomiting is recognised for the nasal spray in kids, and diarrhea is on no list at all.
What the inserts listAdults are barely asked
| Vaccine (2026–27 insert) | Group | GI row in the solicited table | Vaccine vs comparator |
|---|---|---|---|
| Flublok (recombinant) | Adults 18–49 | Nausea | 6% vs 5% placebo |
| Fluad (adjuvanted) | Adults 65+ | Diarrhea / nausea / vomiting | 4.8% vs 4.5% · 2.9% vs 2.8% · 1.4% vs 1.7%, vs another flu vaccine |
| Fluzone, Fluzone High-Dose, Flucelvax, Fluarix, FluLaval | Adults | No nausea, vomiting or diarrhea row is solicited at all | |
| Flucelvax | Infants 6–23 months | Diarrhea | 23.2% vs 20.2%, vs another flu vaccine |
| Fluarix Quadrivalent | Infants 6–35 months | Loss of appetite | 14% vs 15%, vs a non-flu vaccine |
| FluMist (nasal spray) | Children 2–6 | Decreased appetite | 21% vs 17% placebo |
The first thing the inserts reveal is an absence. For adults, five of the eight vaccines on this year’s list — Fluzone, Fluzone High-Dose, Flucelvax, Fluarix and FluLaval — did not solicit a single gastrointestinal symptom in their adult trials. Their diary cards asked about injection-site pain, headache, fatigue, muscle aches, fever. If you were looking for an adult diarrhea percentage for any of those five, it does not exist, which is itself the finding: the trialists did not consider it a likely enough reaction to ask.
Where adults were asked, the numbers are placebo-shaped. Flublok’s insert is the cleanest, because its trial in 18-to-49-year-olds used a true placebo: nausea in 6% of 2,272 vaccine recipients and 5% of 2,231 placebo recipients. Fluad’s insert, for adults 65 and over, did solicit diarrhea — 4.8% in the Fluad group against 4.5% in a group given a different flu vaccine, with nausea at 2.9% versus 2.8% and vomiting at 1.4% versus 1.7%. In other words, roughly one in twenty older adults reported some diarrhea in the week after either shot, and the shot did not change that number. That is what a background rate looks like.
Five of the eight adult inserts never asked about diarrhea. The three that asked about anything gastrointestinal found the same rate in the comparison group.
The number that travelsEighteen percent, and what it is attached to
If you have seen a flu-shot diarrhea statistic online, it is probably Flucelvax’s. The insert’s summary reads: “In children 6 months through 3 years of age… the most common systemic adverse reactions were irritability (28%), sleepiness (27%), diarrhea (18%) and change of eating habits (17%).” Three things are attached to that 18% that tend to fall off in transit. It is toddlers, not adults — the adult tables have no diarrhea row. It is a pooled headline; the actual table shows 23.2% in 6-to-23-month-olds and 14.8% in 2-to-3-year-olds. And the comparison group, given a different flu vaccine, reported 20.2% and 14.0%. Another insert with a toddler diarrhea row shows the same picture from the other side: 24.2% versus 25.6%. Toddlers on a diary card have loose stools about a quarter of the time in any given week. The shot is not what is producing that number; the diary card is.
The nasal spray is the one product with a placebo-controlled gastrointestinal signal in children, and it is appetite, not the bowel: FluMist’s insert reports decreased appetite in 21% of 2-to-6-year-olds versus 17% on placebo, and abdominal pain as an unsolicited event in 2% versus 0%; no vomiting or diarrhea row appears in that table, though both are listed among post-marketing reports. FluMist is also, since 2024, the vaccine adults 18 to 49 can order for self-administration at home — the insert is explicit that children should not self-administer.
The bigger studiesUncertain, and not different
Beyond the inserts, two large sources say roughly the same thing in different registers. The Cochrane review of flu vaccines in healthy adults — 52 trials, more than 80,000 people — found that inactivated vaccines raise the rate of fever from 1.5% to 2.3%, and concluded, in its own words, that “rates of nausea and vomiting are uncertain,” because the trials mostly did not measure them. And the largest dedicated look at gut events, an analysis of 31,983 adults 65 and over randomised to high-dose or standard-dose vaccine across two seasons, found serious gastrointestinal events at 2.44 versus 2.69 per 1,000 participant-seasons — a relative risk of 0.91 with a confidence interval spanning 1, meaning no difference. Five of its six authors work for the manufacturer, which is worth knowing; the finding is a null.
One more piece of context, from a different vaccine entirely. In a 2022 analysis of 12 placebo-controlled COVID vaccine trials, 35% of people who received a saline injection reported a systemic symptom afterwards — headache and fatigue most often. Gastrointestinal symptoms were not broken out and this is not flu data, so treat it as illustration rather than evidence: any week in which a person has had a needle in the arm is a week in which ordinary symptoms get attributed to it. The toddler diary cards above are the flu-specific version of the same effect.
The searches with no sourceGreen poop, and the wrong flu
Two of the autocomplete phrases deserve a direct answer. “Flu shot green poop” has no basis in any document: the words green and bowel appear in none of the nine package inserts, and the only stool references anywhere in them are the definitions of severe diarrhea used to grade the toddler diary cards. Green stool has its own ordinary explanations, none of them vaccines. And “stomach flu” is the source of a lot of confusion: CDC’s norovirus page notes that norovirus “is sometimes called the ‘stomach flu’… However, norovirus illness is not related to the flu.” Influenza itself is a respiratory illness; CDC’s symptom page says vomiting and diarrhea can occur with it but are “more common in children than adults.” The shot does not prevent norovirus, and it does not cause it.
The reading, then: nausea in a small share of adults, matched by placebo where anyone measured it; diarrhea on no CDC list and in no adult table; a toddler number that equals its comparison group; a large null on serious gut events; and a Cochrane verdict of “uncertain” because the question was rarely asked. A stomach that is off in the days after a flu shot is far more likely to be one of the ordinary September things — the new routine, the seasonal drink, the cold going around the office — than the shot. For the record, this season CDC has said the July 2025 flu vaccination recommendations “remain in effect” while its formal advisory process is unsettled, and every insert linked above ends the same way we will: questions about whether and when to be vaccinated belong with a clinician or pharmacist. This article is educational, describes what the cited labels and studies say, and is not medical advice.