Standard rules say that in order the sample to be "fasting", you must not have had anything to eat or drink from 8-12 hours. At least 8 hours, but not more than 12, or you will run the risk of dehydration or fainting. To help counteract that, make sure you drink plenty of water. This will not in any way jeopardize your test results and will make your veins much easier to find. You are able to take any medications, but refrain from any insulin or other glucose-lowering medication until you have eaten.