Adults who have at least one diet drink a day are three times more at risk from a stroke or dementia, research shows today.
Scientists say they should no longer be regarded as the healthier alternative and urge the public to stick to water or milk.
Their study of almost 4,400 adults also suggests diet drinks are more likely to cause strokes and dementia than those full of sugar.
There was no link between sugary beverages and either of the illnesses - although the researchers aren't encouraging us to drink them either.
The team of scientists from Boston University believe the artificial sweeteners including aspartame and saccharine maybe affecting the blood vessels, eventually triggering strokes and dementia.
Diet drinks account for a quarter of the sweetened beverages market but there is growing evidence they are not as healthy as previously thought.
A major review in January by Imperial College London researchers found they were no better at aiding weight loss than full fat drinks.
In fact the authors suggested they were encouraging obesity by triggering the sugar receptors in the brain, making us crave sweet food.
In this latest study - published in the American Heart Association's journal Stroke - researchers looked at 4,372 adults over the age of 45.
They had filled in detailed questionnaires on their food and drink intake in the 1990s and were then tracked for ten years.
The results showed that adults who had one or more diet drink a day were 2.9 times more likely to develop dementia and 3 times more at risk of strokes compared to those who virtually none at all.
Matthew Pase, senior fellow in the department of neurology at Boston University School of Medicine, said: 'Our study shows a need to put more research into this area given how often people drink artificially sweetened beverages.
"Although we did not find an association between stroke or dementia and the consumption of sugary drinks, this certainly does not mean they are a healthy option.
"We recommend that people drink water on a regular basis instead of sugary or artificially sweetened beverages."
Even if someone is three times as likely to develop stroke or dementia, it is by no means a certain fate.
"In our study, 3 percent of the people had a new stroke and 5 percent developed dementia, so we're still talking about a small number of people developing either stroke or dementia."
Rachel Johnson, past chairwoman of the American Heart Association's Nutrition Committee and professor of nutrition at the University of Vermont, said: "We know that limiting added sugars is an important strategy to support good nutrition and healthy body weights, and until we know more, people should use artificially sweetened drinks cautiously.
"They may have a role for people with diabetes and in weight loss, but we encourage people to drink water, low-fat milk or other beverages without added sweeteners."
The researchers are still not sure whether diet drinks are causing strokes or dementia – or whether those who consume them are at higher risk anyway.
Previous studies have shown they tend to be consumed by adults who are already overweight or obese.
Dr Rosa Sancho, Head of Research at Alzheimer's Research UK, said: 'As people are becoming more aware of the consequences of a high-sugar diet, many are turning to artificially-sweetened diet fizzy drinks as an alternative to those with lots of sugar.
"This interesting new study has pointed to higher rates of dementia in people who drink more artificially-sweetened drinks, but it doesn't show that these drinks are the cause of this altered risk.
"Future studies will need to confirm these findings in other groups of people, and explore what might be underlying any link between artificially-sweetened soft drinks and dementia.'
But Gavin Partington, director-general of the British Soft Drinks Association, the industry body said: 'Despite their claims, the authors of this observational study admit they found no cause and effect and provide no science-based evidence whatsoever to support their theories.
"In fact, based on the evidence, Public Health England is actively encouraging food and drink companies to use low-calorie sweeteners as an alternative to sugar and help people manage their weight.
"Surely we should be trying to help consumers reduce their calorie intake, not presenting unproven claims?"
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