Rumors say that taking large doses of vitamin b1 - Thiamine - helps to ward off mosquito bites. Unfortunately, actual tests show that the mosquitoes don't care about your state of health.
One such email says "Bob, a fisherman, takes one vitamin B-1 tablet a day April through October . He said it works. He was right. Hasn't had a mosquito bite in 33 years. Try it. Every one he has talked into trying it works on them. Vitimin B-1 ( Thiamine Hydrochloride 100 mg.)"
First, I wouldn't trust someone that misspells "vitamin" and who relies on Bob the Fisherman for medical advice.
Don't get me wrong. B-1 is an important vitamin and is good for your heart and nervous system. But the US RDA for B-1 is 1.5mg. To take 100 times that amount daily because of an urban legend is really unwise.
If it were TRUE it might be worth looking into the medical risks and tradeoffs. But since research studies show that taking B-1 has no impact at all on mosquito bites, it's best to stay with healthy doses of vitamins. There are many times that too much vitamin is far more dangerous than too little.
Anti Mosquito Information
West Nile Virus Information