Thiamine (Vitamin B1) is said to have insect repellent properties if taken in large doses. People used to take it orally in tablet form, but nowadays it's more usual for it to be absorbed through the skin via a patch. Most independent studies seem to conclude that it doesn't actually work very well as an insect repellent so it should never be relied upon when there is any risk of catching an insect bourne disease, but in less hazardous environments if it works for you then that's just fine!
