很多人以為口服維他命B1、大蒜或酵母錠可以驅趕蚊子，但這是真的嗎？如果無效，那麼將之作為唯一防護措施的人們則有可能感染登革熱或瘧疾等由蚊子傳播之疾病。台大昆蟲系副教授薛馬坦想藉由探討每一篇相關的論文來驗證這個迷思。該結果則於最近發表於著名的昆蟲學期刊 Bulletin of Entomological Research 上，其中涵蓋了巴西、挪威等國家近八十年的研究，並得出了一個明確的結論：口服驅蟲劑並無效用，但仍有很多人，其中不乏醫生，沒能認清這事實。
維生素B1，又稱硫胺素，是世界上最常被誤用為口服驅蟲劑的成分。1943 年，美國的一位醫生聲稱，僅需10 毫克的劑量即能提供長達一整個夏天的防蚊效果。其他一些醫生也提出了類似的說法，但所需的劑量卻不斷提升，甚至高達1000 毫克。當研究人員開始調查，他們發現硫胺素根本沒有防蚊的效果。對照實驗也反覆驗證這樣的結果。硫胺素不論是以藥丸、注射、乳霜甚至是貼片的形式使用，與對照組比較之下，都無法阻止蚊子降落在人類皮膚上叮咬。
為什麼硫胺素沒有用呢？因為蚊子不怕維他命，硫胺素也不會影響汗水或皮膚。人體會謹慎地調控硫胺素的量，不但會藉由尿液來排除多餘的硫胺素，也會阻止腸胃道吸收。任何超過5 毫克的劑量通常不會被消化，並成為廢棄物。那為什麼人們認為它有效？因為蚊蟲叮咬的症狀會隨時間而有所變化。持續接觸同一批蚊子，會使人們不再會有持續數天的延遲反應，而只會有持續幾分鐘的小反應。到最後就算被咬，也不會有任何反應了。這也是為什麼兒童對蚊蟲叮咬的反應通常比成年人強。早期的學者可能誤將這種無症狀的蚊蟲叮咬與驅蟲性搞混了，這就是對照實驗如此重要的原因。
1985年，美國食品藥物管理局（FDA）宣布口服驅蟲劑全部無效。不幸的是，此類產品仍然存在，更不乏聲稱可以透過聲音驅蟲的裝置及手機應用程式。世界各地的醫學協會、旅遊指南及病蟲害管理組織皆已表明上述做法無效，並宣導民眾採取安全有效的驅蟲辦法，如含有DEET（待乙妥）的防蚊液、穿戴長袖或防護網等物理性防護，但許多人仍認為口服驅蟲劑有效，其中不乏醫生和藥師。儘管科學界早已發現維他命B1無法驅蟲，仍須致力宣導這一正確觀念。
Many people believe taking vitamin B1, garlic, or yeast pills can make one’s body repel mosquitoes, but is this true? Such oral repellents would be a convenient alternative to repellents that have to be rubbed on the skin if they work. If they do not work, then people who take them as their only protection would be at risk of diseases spread by mosquitoes such as dengue or malaria. NTU Entomology Professor Matan Shelomi wanted to test this myth by reviewing every paper every written on the subject. The results, recently published in the prestigious Bulletin of Entomological Research, covered nearly eighty years of research in countries from Brazil to Norway, and had a clear conclusion: oral insect repellents do not work, but still many people, including some doctors, do not know this fact.
Vitamin B1, also called thiamine, is the product most commonly misused worldwide as an oral repellent. In 1943, a doctor in the USA claimed that doses as low as 10 milligrams could keep someone protected against mosquitoes for an entire summer. A few other doctors made similar claims, though the necessary dosages kept going up, reaching as high as 1000 milligrams. As researchers began to investigate, they found that thiamine just does not work. Controlled experiments repeatedly failed to show any effect. Thiamine has been tested as pills, as injections, and even as skin creams and patches, but never stops mosquitoes from landing on people or biting them when compared with a control.
Why does thiamine not work? Mosquitoes are not afraid of vitamins, and thiamine does not affect sweat or the skin. The human body controls thiamine levels carefully, not only eliminating extra thiamine quickly in the urine, but also by stopping absorption in the gut. Any dose over 5 milligrams will probably not be digested, and becomes waste. Why did people think it works? Mosquito bite symptoms change with time. With repeated exposure to the same population of mosquitoes, people stop having delayed reactions that last several days, and only have small reactions that last a few minutes. Eventually people have no reactions at all, even though they are getting bitten. This is why children often have stronger responses to mosquito bites than adults. It is possible early researchers confused asymptomatic insect bites with repellency, which is why controlled experiments are so important.
In 1985, the USA Food and Drug Administration declared that oral insect repellents are all ineffective. Unfortunately, such products still exist, along with other ineffective scams like devices that repel mosquitoes with sound, or even mosquito-repelling APPs. None of them work. Medical associations, travel guides, and pest management organizations worldwide have issued statements warning people that they do not work, and to instead use safe and effective repellents like DEET or physical barriers like nets and long sleeves. Unfortunately, many people still think oral repellents work, including some doctors and pharmacists. Even though the scientific community determined that vitamin B1 is not a repellent long ago, work is needed to spread this information.
