Thank you for your inquiry.
You are correct in that there have been anecdotes indicating that thiamine might be helpful as a preventive for insect bites, specifically mosquito bites. However, there is no credible evidence in the literature that supports this contention. Below are two examples of references that refute the claim that thiamine might be helpful in this regard.
Thank you again for your inquiry and we hope this response is helpful to you.
J Am Mosq Control Assoc. 2005 Jun;21(2):213-7.
Testing vitamin B as a home remedy against mosquitoes.
Ives AR, Paskewitz SM; Inter-L&S 101; Biology Interest Groups; Entomology Class 201.
Department of Zoology, University of Wisconsin, Madison, WI 53706, USA.
Vitamin B is often recommended in the popular media as a systemic repellent against mosquitoes. This information is especially prevalent on the Web. The results of a small number of published studies suggested that vitamin B complex supplements are not effective as repellents, but these studies were limited by the use of very few human subjects and only 1 species of mosquito, Aedes aegypti. We extended this work with the use of larger samples of human subjects and with Anopheles stephensi as the test organism. We tested whether ingestion of vitamin B supplements under various regimens affected the attractiveness of volatile skin components transferred to glass vials. Although there was substantial and consistent individual variation in attractiveness, we found no effect of vitamin B supplementation.
Trans St Johns Hosp Dermatol Soc. 1969;55(1):99-102.
Vitamin B1 is not a systemic mosquito repellent in man.
Khan AA, Maibach HI, Strauss WG, Fenley WR.
Phil Lieberman, M.D.