Caffeine is the world's most popular drug, but we're often told we need to avoid having too much.

One of the reasons some of us try to limit the amount the coffee or tea we drink is that we've heard it can be dehydrating because it's a diuretic.

A diuretic is a substance that causes your body to produce urine, and it's been suggested caffeine can do this because it increases blood flow through your kidneys.

But if you regularly enjoy a few cups of coffee or tea a day, the moderate amount of caffeine they contain won't cause you to lose more fluid than you ingest, says dietitian and nutritionist Lisa Renn.

Nor will your cuppa be any more likely to send you off to the loo than any other drink.

"There is evidence that caffeine in higher amounts acts as a diuretic in some people, but moderate intake is actually not that significant," she says.

A 2014 UK study of men who regularly drank coffee found no difference in hydration levels between those who drank four 200ml cups of coffee a day and those who drank the same amount of water.

Researchers measured the men's urine output over a 24-hour period together with other hydration markers in their blood, and concluded moderate coffee intake provided similar hydrating qualities as water.

While the study focused on the intake of coffee, those who drink tea can also take heart from the results because tea contains similar amounts of caffeine as coffee.

It's worth noting this study looked at men who regularly drank coffee. There's some suggestion coffee may have more of a diuretic effect on those who don't normally drink coffee because they haven't developed a tolerance to caffeine.

However, so far the evidence around this isn't completely clear.

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How caffeine affects hydration

If you drink coffee regularly and don't drink too much it shouldn't dehydrate you.

One of the reasons drinks containing caffeine — coffee, tea, cola and energy drinks — have been given such a bad rap over the years is because caffeine is a diuretic when consumed in large doses of more more than 500mg.

Diuretics do make your body produce more urine, meaning you lose sodium and water.

When you lose too much sodium and water, you become dehydrated, and this can have an effect on a range of bodily functions — from temperature control to absorption of food.

However, the amount of caffeine you get in a cuppa is unlikely to have these effects and can actually contribute to your overall daily fluid intake.

"If you have to have more than four cups of coffee a day you may see a diuretic effect from that, but if your intake is less, then from a dehydration view you're going to be OK," Ms Renn says.

Men's bodies need around 2.6 litres and women's around 2.1 litres of water a day which can be gleaned from a range of food and drinks other than water, including coffee and tea.

"Certainly, you can be interspersing coffees and waters throughout the day, so you might have a bottle of water with you and be sipping on that most of the time, and then you might have a coffee in your break times," Ms Renn says.

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The good and bad sides of coffee

Drinking coffee can have a range of health benefits and has been linked to a decreased risk of some cancers, heart disease and type 2 diabetes.

"Coffee is certainly not the evil we once thought it was, but it's that old adage of 'Everything in moderation'," Ms Renn says.

Excessive consumption of caffeine can leave you with side effects that include:

Rapid heartbeat (palpitations)

Rapid heartbeat (palpitations) Restlessness and excitability

Restlessness and excitability Anxiety and irritability

Anxiety and irritability Trembling hands

Trembling hands Sleeplessness

If you're drinking coffee with lots of milk and sugar, that may also affect your weight.

"Lattes and cappuccinos can make an excellent dairy-based or soy-based snack," Ms Renn says.

"But if you're having lots of those throughout the day, especially if you're trying to lose weight, it's going to impact on your calorie intake."

This is general information only. For detailed personal advice, you should see a qualified medical practitioner who knows your medical history.

This story, which was originally published by ABC Health and Wellbeing, has recently been updated.