Are onions toxic
Executive summary
There is no credible evidence that ordinary onions become “poisonous” after being cut; multiple fact-checks, food-safety experts and industry groups say cut onions do not uniquely promote foodborne bacteria and can be refrigerated safely for days (National Onion Association, Center for Food Safety; Full Fact) [1] [2] [3]. However, onions contain natural sulfur compounds that cause eye irritation and, in concentrated form, can be toxic to pets (n‑propyl disulfide) and some individuals may react to FODMAPs — so “toxic” depends on species and context, not a blanket danger from cut onions [4] [5] [6].
1. The viral “cut‑onion is poisonous” claim and where it came from
An oft‑shared social post that warns cut onions become “highly poisonous” overnight appears to be a long‑running internet meme dating back years; researchers and debunkers have traced the claim to unreferenced blog posts and social media, not to scientific studies [3] [7]. Fact‑checking organisations and health blogs note the alarmist wording — “creates toxic bacteria” or “absorbs bacteria” — lacks provenance and is contradicted by food‑safety experts [3] [8].
2. What food‑safety experts and onion authorities say
The National Onion Association and university food‑safety groups state that juice from cut onions can inhibit or kill some microorganisms and that onions do not uniquely promote pathogen growth; the Center for Food Safety at the University of Georgia is cited as saying cut‑onion juice can inhibit several microorganisms [1] [2]. Reviews of refrigerated storage conclude fresh‑cut onions can be kept safely for about a week and that 10–14 days is feasible under controlled refrigeration, undermining the “one night = poisonous” narrative [3].
3. Why people still blame onions for stomach upsets
Onions are common in foods that cause foodborne outbreaks (e.g., salads, deli mixes), so they are easy scapegoats when people get sick; however, experts point out contamination risks are usually environmental — dirty hands, contaminated cutting boards, or cross‑contamination — not an intrinsic trait of the onion itself [1] [9]. Independent reviewers emphasize there is “no proof onions can absorb bacteria” and that sensational social posts conflate correlation with causation [8] [3].
4. Natural onion chemistry and non‑infectious harms
Onions produce sulfur‑containing compounds that make you cry (the “lachrymatory factor”) and these same sulfur compounds have antimicrobial activity; older chemical reviews list thiopropanal S‑oxide as the lachrymatory agent and note naturally occurring sulfur chemicals exist in the Allium family [4] [10]. That chemistry explains both onion’s strong smell and some of its preservative effects cited by food‑safety sources [1] [2].
5. Who can be harmed by onions — and how
Available reporting shows humans generally tolerate culinary amounts of onions, but onions can cause digestive symptoms in people sensitive to FODMAPs and they can be seriously toxic to dogs and cats because of n‑propyl disulfide, which causes oxidative damage to pet red blood cells [6] [5]. Veterinary and toxicology sources warn that doses far smaller relative to body weight can harm pets, even though human red blood cells are more resistant [5].
6. Practical, evidence‑based guidance
Treat cut onions like any fresh‑cut produce: refrigerate promptly in a sealed container, avoid cross‑contamination with raw meats, and use within the recommended refrigerated shelf life (about 7–14 days by the National Onion Association and review sources) [3] [1]. If a cut onion smells off, shows visible spoilage, or sat out at room temperature for many hours in unsafe conditions, discard it — those are standard food‑safety triggers cited by experts [1].
7. What the sources don’t say or leave uncertain
Available sources do not present any peer‑reviewed evidence that cut onions become intrinsically “poisonous” after a single night; they also do not claim onions are sterile or immune to contamination from external sources [3] [1]. There is limited quantitative data in these summaries about precise microorganism suppression by onion juice, so the degree and range of antimicrobial effects are described qualitatively in current reporting [2] [1].
Bottom line: the headline claim that cut onions turn poisonous overnight is not supported by the cited expert, industry and fact‑check sources; onions have biologically active compounds that can irritate or harm certain species and sensitive people, but the everyday food‑safety risk from storing cut onions properly is low [3] [5] [6].