A fasting blood test means you stop eating and drinking calories for a set number of hours, usually 8 to 12. Water is fine and, for most people, encouraged. Tea, coffee, juice and chewing gum are not, because they can move glucose and triglycerides.
Take your regular medicines unless the prescribing doctor has told you to hold them. If you take thyroid tablets, ask whether to delay the morning dose until after the draw. Insulin and other diabetes medicines need a specific plan. Do not skip them on your own.
Book an early slot so the fast happens mostly while you sleep. Keep the phlebotomist's number handy. If you feel faint, sit down, say so, and the collector will pause.
After the sample, eat the breakfast you packed. Fasting results are a snapshot. Your doctor reads them next to HbA1c, symptoms and previous reports.
This article explains how tests are used. It is not a personal diagnosis.