The science behind double dipping
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Double-dipping, a common habit where someone dips a chip, takes a bite, and dips again, has long been viewed as unsanitary. Research shows that double-dipping can transfer bacteria from the mouth to the dip, increasing the microbial load and potentially spreading illness, especially in communal food settings. While the risk of spreading germs may be small for healthy individuals, avoiding the practice is a simple step to promote hygiene and protect others, especially during cold and flu season.
Source
TREVINO, JUDITH & BALLIEU, BRAD & YOST, RACHEL & DANNA, SAMANTHA & HARRIS, GENEVIEVE & DEJONCKHEERE, JACKLYN & DIMITROFF, DANIELLE & PHILIPS, MARK & Han, Inyee & MOORE, CHLOE & Dawson, Paul. (2009). Effect of biting before dipping (double-dipping) chips on the bacterial population of the dipping solution. Journal of Food Safety. 29. 37 - 48. 10.1111/j.1745-4565.2008.00137.x.
Additional Reading
https://www.health.harvard.edu/blog/double-dipping-chip-dangerous-justicky-2016080410059
https://www.washingtonpost.com/wellness/2025/05/28/double-dipping-germ-risk/