Fact CheckHealth

Are left over onions poisonous to humans?

Claim: A Facebook user claims onions absorb bacteria and cut onions are poisonous. 

Verdict: Our findings show there is no proof onions can absorb bacteria or cut onions are poisonous.

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A Facebook post by Kunle Alade claims that onions absorb bacteria which is why they help prevent colds and flu. In the post, Mr Alade claimed this was discovered during the flu pandemic in 1919, by an unnamed doctor 

“Onions ABSORB BACTERIA* and that is the reason why they are so good at preventing us from getting colds & flu,” part of the post read. 

The post advised against eating an onion that has been sliced and left open for a while because they are poisonous. 

Are left over onions poisonous to humans?
Screenshot of the Facebook post. 


We looked at onion and its properties and also carried out a keyword search.  Onions known as allium cepa, are bulb-shaped vegetables that grow underground. They vary in size, shape, colour, and flavour but the most common types are red, yellow, and white onions.

A keyword search led to an analysis by McGill Office for Science and Society, which showed onions are not bacteria prone or bacteria magnets because they have a variety of sulphur compounds with antibacterial activity.  

This report also noted that sliced onions left open are not poisonous unless it was sliced on a contaminated cutting board, or handled with dirty hands. This can, however, be prevented with good food practices like washing of hands, kitchen tools and wiping down counters with sanitizer before beginning food preparation, to avoid introducing outside contamination.

This practice extends to the preparation of every other food item, not just onions. 

The report also noted that contamination by some sort of mould is possible, because mould spores can travel through the air. 

READ ALSO: No evidence mixing kola nut, lime, ginger, others cures menstrual problems

Cookinglight article also disagrees with the claim that cut onion is poisonous.

We found an article by the National Onion Association (NOA) which debunked several myths about onions including the claim that cut onions are poisonous. The association noted that when handled properly, cut onions are not poisonous and can be stored in the refrigerator in a sealed container for up to 7 days.

A study on the antibacterial action of onion extracts against oral pathogenic bacteria showed onion extracts possess an effect on all bacterial strains.

This article by verywellhealth.com noted there is no proof that onion can help prevent the flu or the common cold as claimed in this post. It also revealed that the idea of keeping raw onions in the room when sick dates back to the 1500s, according to the NOA. 

Our search also led to fact-checks by USA Today, snopes, Wall Street Journal, and Associated Press clarifying that leaving an onion around the house won’t fight or prevent illness/flu because they don’t absorb bacteria.


Our findings show there is no proof onions can absorb bacteria. Placing onions around the house won’t help fight the flu. It is the immune system’s attack on invading viruses or bacteria entering the body that helps the body recover from sickness. It is also not true that cut onions are poisonous as they can even be stored in the refrigerator in a sealed container for up to seven days. The claims are therefore false.

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