Thiamine is vitamin B1. Your body needs it to turn the carbohydrates you eat into usable energy. You can get it from whole grains, legumes, nuts, pork, and supplements.
Inside cells, thiamine helps enzymes process glucose. This keeps your nerves and muscles working. Most people in developed countries get enough from food. Shortfalls can happen with heavy alcohol use, certain diuretics like furosemide, or a diet built around refined carbs. Early signs of a shortage include fatigue, crankiness, and nerve discomfort.
A doctor may recommend a supplement to fix a proven deficiency or to meet higher metabolic demands. Take it with a meal to help absorption, though any time of day works. Large doses usually cause no trouble. Alcohol and some water pills flush thiamine from the body faster, so you might need more.