How to Prepare for a Blood Test: Fasting, Hydration, and Timing

Quick answer

Fast for 8 to 12 hours before your draw. Drink plenty of water. Skip alcohol and hard exercise the day before. Take your usual medicines on schedule, unless told otherwise. Book an early morning slot if you can. Tell your care team about any recent illness, medicines, or supplements.

What "fasting" really means

A fasting blood test means no calories at all. That means no food, no sugary drinks, no milk, no juice, and no alcohol, for 8 to 12 hours before the draw.

Fine during the fast:

• Plain water. Drink it freely.
• Most regular prescription medicines.
• Coffee or tea: usually not allowed, since it can shift some markers. Check your booking instructions.

Not fine during the fast:

• Any food, even a small snack.
• Sweetened drinks, juice, or milk.
• Alcohol.
• Gum or candy, since sweeteners can affect glucose.
• Smoking, which affects several markers.

An 8-hour fast is the shortest that still works well for glucose, insulin, cholesterol, and HbA1c. Twelve hours is the standard target.

Why hydration matters just as much

Many people focus so hard on fasting that they forget to drink water. That is a mistake. Good hydration:

• Makes your veins easier to find, so the draw goes faster.
• Keeps your creatinine and electrolyte readings accurate.
• Lowers your chance of feeling lightheaded after the draw.

Drink water freely the evening before, and have a glass first thing in the morning. Water alone never breaks a fast.

The day before your test

Food

• Eat normally, but skip a heavy or fatty meal that night. It can briefly raise triglycerides and a liver marker called ALT.
• Skip alcohol for 24 to 48 hours. It affects several markers, including GGT and uric acid.

Exercise

• Skip hard workouts, long runs, or heavy lifting the day before.
• A light walk is fine.

Sleep

• Keep your normal sleep schedule. Do not stay up unusually late or wake unusually early.

Medicines and supplements

Regular prescriptions: most people keep taking these on schedule, unless their doctor says otherwise. A few notes:

• Blood pressure medicine: usually taken as normal.
• Thyroid medicine: sometimes held until after the draw, to get an accurate baseline reading. Ask your doctor.
• Diabetes medicine, including insulin: coordinate closely with your doctor here.
• Statins and birth control pills: usually taken as normal.

Supplements

• High-dose biotin, meaning more than 5 mg a day, can throw off several lab tests. Stop it 48 to 72 hours before your draw.
• Vitamin D and iron: keep taking these unless your doctor says otherwise.
• Most other supplements: no need to stop.

Antibiotics or other short-term medicines: mention these to your doctor, since a few can affect results.

Timing matters for some tests

Time of day affects a few markers:

• Testosterone in men peaks in the morning. Test between 7 and 10 AM.
• Cortisol also peaks in the morning. The standard draw time is 8 AM.
• TSH shifts a little through the day, so morning is the usual standard.

Cycle day matters for female hormone testing:

• Day 3 of your cycle for FSH, LH, estradiol, and AMH.
• Day 21 for progesterone.
• Any day works for thyroid, prolactin, testosterone, and SHBG.

Our guide to timing your blood test covers this in more depth. If you are unsure, your booking instructions should say which day to pick.

The morning of your test

Do

• Wake at your normal time.
• Have a glass of water first thing.
• Take your regular medicines on schedule, unless told otherwise.
• Use the bathroom before your draw.
• Wear a top with sleeves that roll up easily.
• Keep a photo ID handy.

Avoid

• Coffee or tea, unless your instructions allow it.
• Smoking.
• Hard activity right before the draw.
• Heavy lifting with your draw arm.

What to share before your draw

Before your draw, mention:

• Any current medicines or supplements.
• Any recent illness, infection, or fever, within the last week or two.
• A recent vaccine, within the past week.
• Your cycle day, if you are testing hormones.
• Any past trouble with blood draws, such as fainting.
• If you broke your fast by accident.
• If you took biotin in the last 48 hours.

Being upfront here protects the quality of your result. Small issues can often be adjusted on the spot, or the affected marker can be flagged for a retest. Our guide to blood draws walks through the full visit.

After your draw

• Eat right away. Break your fast with a balanced meal.
• Drink an extra glass of water.
• Keep the bandage on for 30 to 60 minutes.
• Skip heavy lifting with that arm for 1 to 2 hours.
• Light activity is fine. Save hard exercise for 4 to 6 hours later.

Good prep matters even more with a wider panel. You can book a &you Labs panel and get a report that shows the standard lab range next to a healthy target range, plus a doctor's written plan. See why a &you Labs panel measures more than a standard doctor's test.

Common mistakes to avoid

• Fasting too long. Past 14 hours can affect some markers and raises the chance of feeling lightheaded.
• Forgetting to drink water during the fast.
• Drinking black coffee, thinking it has no calories. It still shifts some results.
• Sneaking a small snack "because it's healthy.".
• Taking biotin within 48 hours of your draw.
• Hard exercise or heavy drinking the night before.

Frequently asked questions

Can I drink water during my fast?

Yes. Drink it freely. Water never breaks a fast.

Can I take my morning medicines?

Most regular medicines are fine on schedule. Common exceptions include thyroid medicine, diabetes medicine, and high-dose biotin, which should stop 48 to 72 hours before.

What if I accidentally ate something?

Tell the person drawing your blood before you start. A few markers, like glucose and triglycerides, may need a redraw. The rest of your panel usually still counts.

Can I brush my teeth before the test?

Yes. Brushing your teeth does not break a fast.

Is a longer fast, like 16 hours, better?

No. The standard 8 to 12 hour fast is set for how the lab tests work. A longer fast does not improve accuracy, and raises your chance of feeling lightheaded.

Key takeaways

• Fast 8 to 12 hours, water only. Skip alcohol for 24 to 48 hours before.
• Stay well hydrated through the fast. Water does not break it.
• Keep taking regular medicines on schedule, unless told otherwise, and stop high-dose biotin 48 to 72 hours before.
• Time of day matters for testosterone, cortisol, and TSH. Cycle day matters for female hormones.
• Mention recent illness, medicines, or a broken fast before your draw. Honesty protects your result.