Vitamin B1 does not break down in the body and has no known effect on skin. The body strongly regulates it, absorbing little ingested thiamine after the first 5 milligrams and quickly excreting any excess via urine, so it does not build up. Overdose is almost impossible.

As in humans, thiamine is an essential nutrient for mosquitoes. There is no reason they would fear it or try to avoid it. Nor is there evidence that they can smell it.

The best sources of thiamine are whole grains, beans, pork, poultry and eggs. If eating a carnitas burrito won’t make you repel mosquitoes, then neither should a pill.

What explains the early reports, then? Along with shoddy experimental design, many used anecdotal patient reports of fewer bite symptoms as a proxy for reduced biting, which is not a good way to get an accurate picture of what’s going on.

Mosquito bites are followed by two reactions: an immediate reaction that starts fast and lasts hours and a delayed reaction lasting days. The presence and intensity of these reactions depends not on the mosquito, but on your own immune system’s familiarity with that particular species’ saliva. With age and continued exposure, the body goes from no reaction, to delayed reaction only, to both, to immediate reaction only, and eventually no reaction.

What Shannon and others thought was repellency could have been desensitization: The patients were still getting bitten, they just stopped showing symptoms.

Despite the scientific consensus, a 2020 survey of pharmacists in Australia found that 27% were still recommending thiamine as a repellent to patients traveling abroad: an unacceptable recommendation. Besides wasting money, people relying on vitamins as protection against mosquitoes can still get bitten, potentially putting them at risk of diseases like West Nile and malaria.

To get around the American ban and widely agreed-upon scientific consensus on oral repellents, some unscrupulous dealers are making thiamine patches or even injections. Unfortunately, while thiamine is safe if swallowed, it can cause severe allergic reactions when taken by other routes. These products are thus not only worthless, but also potentially dangerous.

Not every problem can be solved with food. Long sleeves and bug spray containing DEET, picaridin or other proven repellents are still your best defense against biting pests.

This article is republished from The Conversation, an independent nonprofit news site dedicated to sharing ideas from academic experts. It was written by: Matan Shelomi, National Taiwan University. Like this article? subscribe to our weekly newsletter.

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Matan Shelomi does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.