The answer, in one line

Plain water is allowed throughout a fasting period, right up to the draw, and no laboratory asks you to stop. Fasting means no food and no drink other than water. Water is the exception, not a tolerated cheat.

It is worth saying plainly because the confusion is widespread. When researchers questioned 150 people arriving at a laboratory, only 39% could define fasting correctly and more than half had been given no instructions at all. If nobody told you, you are in the majority.

Why it helps rather than hurts

Two reasons. The first is practical: a well-hydrated vein is fuller and easier to find, which makes the needle quicker and reduces the odds of a second attempt. Phlebotomy guidance treats patient preparation as part of sample quality, not as a courtesy.

The second is about the result itself. Blood drawn in a dehydrated state is slightly concentrated, which can nudge several values upward: haematocrit, proteins, sometimes sodium. Drinking normally keeps that distortion out of your report. Going without water for twelve hours to be extra safe achieves the opposite of what you intended.

What counts as water, and what does not

Plain water, still or sparkling, at any temperature. That is the whole list.

Everything else breaks the fast, including things that feel like water. Flavoured water, water with a slice of lemon, herbal tea, black coffee without sugar, sugar-free soft drinks and chewing gum all introduce something the laboratory did not expect. Black coffee is the most common mistake, and it is not a trivial one: in controlled trials, caffeinated coffee measurably worsened the glucose and insulin response compared with decaffeinated coffee or water.

Medicines are a separate question. Take your usual treatment with water unless the prescriber told you otherwise in writing.

How much, and when

There is no target to hit. Drink as you normally would, including a glass or two on the morning of the appointment. You are not trying to load up, only to avoid arriving dry after a night without fluids.

One practical note if you are prone to feeling faint during a draw: arriving well hydrated and having eaten the evening before makes that less likely. Tell the person taking the sample if it has happened to you, before they start.