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Vitamin B1 is Not a Mosquito Repellent. So Why Do Doctors Prescribe It?

Anopheles mosquito

Some faulty science in the 1940s gave rise to the myth that consuming vitamin B1 could deter mosquito bites. A new review compiles the decades of scientific studies disproving the idea and explores why it still persists. Shown here is an Anopheles sp. mosquito. (Photo by Insects Unlocked via Flickr, public domain)

By Matan Shelomi, Ph.D.

Matan Shelomi, Ph.D.

Matan Shelomi, Ph.D.

Many people believe taking vitamin B1, garlic, or yeast pills can make their body repel mosquitoes without the need for repellents or nets or long sleeves. Is there truth to this? With all the unfounded fearmongering over the safe and effective DEET, my initial response when hearing a friend-of-a-friend extol the superpowers of supplements as “systemic repellents” was to assume this was yet more medical pseudoscience. Spoiler alert: It is indeed bunk. Still, I had to do my due diligence and find the evidence for or against my view, but to my surprise there were no review papers on the question of edible insect repellents.

So, I wrote one myself.

My review paper, published earlier this year in the Bulletin of Entomological Research, has, for the first time, identified the scientific consensus on the matter of vitamin B1 and insect repellency. It covers over 100 papers from multiple countries in a half dozen languages spanning nearly 80 years, including papers unavailable online from journals that no longer exist. (Hooray for interlibrary loan!) The conclusion is unequivocal: Oral mosquito repellents do not work. So, where did this idea come from, and why won’t it go away?

The myth started in a 1943 case report by a Minnesota pediatrician who claimed oral doses of vitamin B1, or thiamine, as low as 10 milligrams (mg) could protect one from bites for an entire summer. Attempts a year later by the U.S. Naval Medical Research Institute, Department of Agriculture, and the National Institute of Health to replicate the results failed, but the myth had already taken root, with doctors and veterinarians recommending ever larger doses of thiamine (up to 1,000 mg daily) to protect humans and animals from mosquitoes, fleas, and ticks.

vitamin B1 bottle

In all properly controlled experiments that have studied it, vitamin B1 has failed to have any effect on pest landing, probing, or biting rates. In fact, despite extensive searches, no food, supplement, or drug of any kind has ever been shown to make anyone repel any insects. The explanations for why vitamin B1 fails to repel mosquitoes are numerous, starting with the fact that vitamin B1 is nutritious for mosquitoes too, so there is no reason they would fear it. (Photo by Matan Shelomi, Ph.D.)

While some old case reports and uncontrolled studies in predatory journals claim otherwise, in all properly controlled experiments vitamin B1 failed to have any effect on pest landing, probing, or biting rates. In fact, despite extensive searches, no food, supplement, or drug of any kind has ever been shown to make anyone repel any insects. As desirable as they would be, systemic repellents remain the stuff of legend. Every relevant entomological and medical organization to issue statements on the subject since the 1990s has agreed that oral repellents are a myth.

The explanations for why thiamine fails to repel mosquitoes are numerous, starting with the fact that vitamin B1 is nutritious for mosquitoes too, so there is no reason they would fear it. Pharmacological studies found that absorption of swallowed vitamin B1 is greatly reduced after the first 5 mg. The human body does not store vitamin B1 long-term, but rather it excretes excess quickly through the urine—meaning large doses will have no effect on thiamine levels in the body. People with thiamine deficiencies, meanwhile, are not more attractive to mosquitoes, nor do people taking thiamine to treat deficiencies become less attractive. Experiments confirmed neither oral nor topical thiamine affects the skin, skin bacteria, or body odor in any way. In theory and in practice, vitamin B1 completely fails as an insect repellent, as does thiamine-rich brewer’s yeast, another alleged repellent.

Why did early doctors mistake thiamine as repellent? These reports typically used reduction in bite symptoms as a proxy for reduced insect bites, rather than directly observing biting mosquitoes. The problem is that intensity of bite reaction and attractiveness to mosquitoes are uncorrelated. Mosquito-bite reactions have two parts: an immediate reaction that appears within minutes and vanishes within hours and a delayed reaction that emerges later and lasts for days. Whether and how strongly one experiences either reaction changes with age and with continued exposure to bites, until eventually one has no reactions at all despite being bitten. This phenomenon was only described in the late 1980s, so the early doctors likely mistook the natural progression of individuals’ mosquito-bite reactions for a reduction in bites.

Despite being thoroughly debunked, the myth that some oral supplements can make people invisible to mosquitoes is still popular today. Surveys of pharmacists in Switzerland and Australia found that a fifth were recommending vitamins as insect repellents for people travelling abroad, unnecessarily exposing their patients to a risk of contracting mosquito-borne diseases like malaria or dengue. An explanation for this persistence is that doctors and pharmacists may likely enter medical school already believing the myth since childhood and would need to be explicitly taught that it is false. The internet is not helping, as vitamin-based repellents are promoted on the same types of websites selling other, equally worthless alternative or “natural” remedies. Because the body excretes consumed thiamine so readily, overdose is practically impossible, which makes vitamin B1 a safe ingredient for snake-oil sellers to put in their placebos.

The myth has also evolved since the 1940s and not just in the alleged dosages needed for an effect. While the U.S. Food and Drug Administration banned the marketing of oral insect repellents in 1985 for being fraudulent, new products attempt to circumvent this restriction by formulating extremely high doses of vitamin B1 into creams, transdermal patches, and even injections. These products are neither safe nor effective, as injected thiamine can trigger severe allergic reactions while still having no effect on biting insects.

The situation is comparable to that of smoking, in which it took several decades before doctors accepted the scientific consensus linking smoking with lung cancer, or to the more recent myth that drugs like hydroxychloroquine or ivermectin can treat COVID-19, despite evidence to the contrary. It takes time for practicing doctors to catch up to the scientific community, if ever, but unfortunately online misinformation spreads faster than the truth.

Read More

Thiamine (vitamin B1) as an insect repellent: a scoping review

Bulletin of Entomological Research

Matan Shelomi, Ph.D., is an associate professor of entomology at National Taiwan University in Taipei. Email: mshelomi@ntu.edu.tw.

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