How to prepare for a blood test: fasting, water and medication

You have been asked for a blood test and the instruction was something like “come fasting”. So far, easy. The problem starts the night before: can I drink water? What about coffee? Should I take my morning tablet?

I will explain it the way I explain it when I prepare an extraction. The summary is this: 8 to 12 hours without food, water only, never stop a medicine on your own (ask first), and no hard exercise or alcohol the day before. Below is the reason for each one.

How many hours of fasting do I need?

The usual is 8 to 12 hours without food or drink, except water. The European reference recommendations for blood collection mention about 12 hours. If you have dinner at nine at night, by nine in the morning you are at the ideal point. That is why blood tests are done early: you sleep through the fast.

Why? Because what you eat passes into your blood and can shift figures such as glucose or triglycerides. If you eat beforehand, the result may be altered and sometimes it has to be repeated.

Note: not every test requires fasting. Glucose and cholesterol usually do; many others do not. Check the request form or ask whoever issued it. And do not fast for longer “just in case”: more hours add nothing and leave some people feeling faint.

Water yes; coffee, tea and gum, better not

Plain water is allowed and actually helps. If you are well hydrated, your veins are easier to see and feel, and the extraction is easier. A couple of glasses when you get up are fine.

Coffee, tea and energy drinks: no, with or without sugar. Nor juices, soft drinks or flavoured water. Not chewing gum either, even sugar-free, and better not to smoke that morning. All of it can alter some values.

What about my medication?

This is the doubt that matters most and the one most often improvised. My rule is simple: never stop taking a medicine on your own.

Laboratory guidelines generally suggest not taking medication before the extraction unless it is necessary. But “necessary” covers a great deal: blood pressure, thyroid, anticoagulants, antiepileptics… If you take something regularly, the safest thing is to ask first, your doctor or me. Often it is taken with a little water and noted on the request form, but that decision belongs to whoever treats you.

Three nuances:

• If the test measures the level of a drug, the time you take it matters. You will be told how to do it.
• If you have diabetes and use insulin or other diabetes medication, do not fast without speaking to your doctor first: there is a risk of low blood sugar.
• Tell me about vitamins and supplements too. They count as medication.

The day before: what almost nobody considers

• Intense exercise, better not in the previous 24 hours. A hard gym session can alter values such as muscle enzymes. Walking and your usual activity are fine.
• No alcohol at all in the previous 24 hours.
• A normal, light dinner, no feast.
• Rest. And if you have a fever or have had an infection these days, say so, because it can affect the results.

The day of the extraction

Get up, drink water and do not have breakfast yet. Wear a T-shirt or a shirt with sleeves that are easy to roll up. If you can, arrive early and sit for about 15 minutes beforehand: rest also affects some results. If you feel faint at the sight of blood or needles, tell me beforehand. We will do it calmly, sitting or lying down.

Before we start I ask your name and date of birth and compare them with the request. It sounds trivial, but it is the most important step: making sure the sample is yours.

When we finish, have breakfast, you have earned it, and keep something at hand in case you feel weak. Press on the puncture point for a few minutes without bending your arm. A small bruise is normal.

If you have eaten or drunk something by mistake, tell me before the extraction. Depending on the test, we either go ahead and note it or change the appointment.

If we do it at home

Doing it at home has a clear advantage: you do not have to go out fasting, drive, park and wait in a room while hungry. I come to you, check the request, prepare and label the tubes with your details and take the sample.

→ See the home blood test service

Informational content. It does not replace the assessment of a healthcare professional or a medical consultation.

Last reviewed: 29 October 2026