Is double-dipping really unhygienic? Scientists reveal the truth about sharing food

From double-dipping crisps to blowing out birthday candles, researchers have tested some surprisingly common food sharing habits to see just how many germs they spread.

The results may change how you think about sharing that snack.

The short answer is that sharing food can spread bacteria and viruses, but for most healthy people the risk from occasional sharing is low.

There are a couple of things to consider though, before you dunk in that communal dip.

Allergies: Sharing food with someone who has allergies could increase their chance of accidental exposure, so avoid sharing to limit contamination risks.

Illnesses and weakened immune systems: If you're unwell, it's safest not to share food and drinks. People who are immunocompromised could be especially vulnerable to infections.

Back in the '90s, in American sitcom Seinfeld, a scene played out which was so iconic that now, 30 years on, it still frequently goes viral on TikTok., external

"You took a bite… and you dipped again! That's like putting your whole mouth right in the dip," cried a horrified character.

But is double-dipping actually gross or just bad manners?

Prof Paul Dawson, a food scientist at South Carolina's Clemson University, measured this exact idea, external and found that dip samples that had been double-dipped had five times as much bacteria ones that hadn't.

While it doesn't sound great, that bacteria decreased over time, and wouldn't pose a big risk to most healthy people.

If you think you can get round the issue with the dip-and-flip method, when you dip the side of the crisp you haven't bitten, you might want to think again.

Dawson told us, "it's probably as objectionable to touch a chip with your hands and fingers before sticking it back into a dip.

"It would depend on the hygiene of the person touching the food, which varies from person to person."

Double-dipping can also be an allergy risk, as someone might eat from a dip that has been contaminated with another sauce containing an allergen.

Back in the 19th century, UK vendors used to sell ice cream in a tiny glass called a 'penny lick', which customers would lick clean before returning.

It was then refilled and served to the next customer – and this is believed to have contributed to the spread of tuberculosis and cholera.

Unsurprisingly, penny licks were outlawed in 1898.

But is sharing an ice cream cone today anything like as risky?

Sharing a tub of ice cream, even with two spoons, is essentially another form of double-dipping, as is licking the same ice cream from a cone. (Yes, it also counts if you share it with your dog.)

So, if you're saying no to a double-dipped crisp, you'll more than likely want to say no to this too.

Sharing a box of popcorn at the cinema is common practice – but is it really a problem?

Another study from Clemson University, external found that relatively small amounts of bacteria are transferred to popcorn when it's handled.

But, it varies considerably from person to person and more bacteria might be transferred at the cinema versus your home sofa.

For most healthy adults, you don't need to worry. But people with a weakened immune system, who are pregnant or unwell, may want to skip sharing this snack.

In another trial, external, Dawson discovered that when candles had been blown out on a cake, bacteria levels on the icing were 14 times higher than on a cake on whose candles hadn't been blown out.

Bacteria varied depending on the person blowing and how enthusiastically they blew.

So does it matter? If someone's ill and blows out candles, you might want to give the cake a wide berth, otherwise you're probably fine.

The five-second rule is seen as law by many, with its implication that if dropped food is picked up quickly enough it won't be contaminated by bacteria from the floor.

Food safety expert Prof Anthony Hilton says it depends on what type of food you've dropped and the type of surface it landed.

He says it's usually worse to eat food that's been dropped on smooth surfaces like hardwood than on rough surfaces like carpet.

Scientists believe that's because smooth, flat surfaces allow more direct contact between the food and the contaminated area, making it easier for bacteria to transfer, external.

Sticky foods like butter, jam and ice cream are most prone to picking up bacteria.

Dr Ronald Cutler has also studied the five-second rule and says that there's no safe amount of time for food to be on the floor.

He says no matter how quickly you pick it up, bacteria will have been transferred – including potentially harmful bacteria.

While experts don't fully agree on whether you should follow the five-second rule – bacteria does transfer instantly. So, do you want the food so badly you'll take the risk?

Not all bacteria is harmful, and most everyday exposures to it don't result in illness.

Risk depends on things like the type of germ, the amount present and the health of the person exposed.

Dawson says, "These studies are not really big food safety issues, but they're interesting and fun. I hope they make people aware of good hygiene, but I don't want anyone to be overly anxious about it."

Originally published April 2019. Updated July 2026.

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