Most fasting blood tests need you to stop eating and drinking anything except water for about 8–12 hours beforehand, but the exact time depends on which tests your doctor ordered.

Quick Scoop

  • Typical fasting window: 8–12 hours with water allowed.
  • Best approach: Schedule the blood work early in the morning so most of the fast happens overnight.
  • Not all blood tests need fasting (for example, many thyroid tests do not), so always follow the instructions on your lab slip or from your clinician.
  • You may be told to avoid alcohol for longer (often 24 hours) before some tests, because it can affect liver, lipid, and other results.

Common fasting times by test (examples)

  • General “fasting blood work” (glucose, many metabolic and lipid panels): usually 8–12 hours.
  • Lipid / cholesterol panel (LDL, HDL, triglycerides): often 9–12 hours, depending on the lab’s protocol.
  • Basic metabolic panel or fasting glucose: commonly 8–12 hours.
  • Iron studies or certain vitamin levels (like B12) may require up to about 12 hours, based on the ordering provider’s instructions.

Practical mini‑guide

  1. Count back 8–12 hours from your appointment time and avoid food and drinks with calories during that window; plain water is usually encouraged so you stay hydrated.
  1. Take your usual medicines only as your clinician advised; some should be delayed, others must not be skipped.
  1. If you accidentally eat or drink something (other than water), tell the lab staff—they may still be able to run some tests, or they may reschedule you.

If you’re unsure which tests you’re getting or have conditions like diabetes, pregnancy, or you take multiple medications, call your doctor or the lab and ask, “Exactly how long should I fast and what’s allowed?”—their specific instructions always override general rules.

Information gathered from public forums or data available on the internet and portrayed here.