With caffeine, it seems that as long as you are not consuming mass amounts of it every day, a little bit every so often is fine and can actually help runners when they need a little boost of energy. However, too much caffeine will leave you dehydrated, and if you become addicted to it, you may feel sluggish and tired when going without it. If you drink high amounts of caffeine, it is wise to make sure you rehydrate yourself.