For years, coffee and tea drinkers have been told to add an extra glass of water for every caffeinated drink, just to “cancel out” the dehydration. It’s one of the most repeated pieces of health advice in Australia — and it turns out the science behind it is much weaker than the warning suggests.
Caffeine does have a mild diuretic effect, meaning it can make you urinate slightly more. But for regular coffee or tea drinkers, the body adapts quickly, and the fluid you take in with your drink largely offsets any extra fluid loss.
Several studies looking at regular caffeine consumers found no meaningful difference in hydration levels between people who drank caffeinated beverages and those who stuck to water alone. The key factors were:
The idea likely traces back to older studies using much higher, one-off doses of caffeine given to people who weren’t regular consumers. Under those conditions, a short-term diuretic effect was measurable — but it doesn’t reflect how most people actually drink coffee or tea day to day.
Moderate caffeine intake from coffee, tea or energy drinks counts toward your daily fluid intake, not against it.
Caffeine sensitivity varies a lot between people, and very high intake can still cause side effects like jitters or disrupted sleep. But purely from a hydration standpoint, there’s no need to add extra water to “make up” for your coffee.
Your daily cup of coffee or tea isn’t working against your hydration goals. Drink water when you’re thirsty, and don’t feel obligated to double up every time you have a caffeinated drink.
