People who consume diet drinks every day are almost three times more likely to suffer a stroke or dementia, research suggests.
Having at least one diet drink a day increased the risk compared to consuming less than one diet drink a week, a study found.
However, researchers found no link between sugary drinks and an increased risk of stroke and dementia, though they warned people not to view sugary drinks as a "healthy option".
Due to the fact that the study is observational and based on food questionnaires, they said further studies are needed on the links between drinks, dementia and stroke.
The new research, published in the American Heart Association's journal Stroke, is based on data for more than 4,300 people taking part in the Framingham Heart Study.
Those in the stroke arm of the study were over the age of 45, while those in the dementia arm were over 60.
All participants filled in questionnaires on their food and drink intake at three separate points during the 1990s.
Researchers then followed the group for 10 years, noting 97 cases of stroke during that period, and 81 cases of dementia (63 cases were specifically Alzheimer's disease).
After adjusting for factors that could influence the results, such as age, sex, education, calorie intake, exercise and smoking, people who had at least one diet drink a day had an almost three times increased risk of dementia or stroke.
The researchers said future studies should look at the effect of diet drinks on factors known to increase the risk of stroke and dementia, such as high blood pressure.
"As the consumption of artificially sweetened soft drinks is increasing in the community, along with the prevalence of stroke and dementia, future research is needed," they added.
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."
He added: "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% of the people had a new stroke and 5% 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."
Gavin Partington, director-general of the industry-funded British Soft Drinks Association, 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?"
Dr Rosa Sancho, head of research at Alzheimer's Research UK, said: "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.
"When the researchers accounted for other risk factors for Alzheimer's, such as risk genes, diabetes, heart disease, cholesterol levels and weight, this significant association was lost, suggesting that these drinks are not the whole story.
"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."
Dr Elizabeth Coulthard, consultant senior lecturer in dementia neurology at the University of Bristol, said: "Although interesting, this paper does not tell us that artificially sweetened drinks cause stroke or dementia.
"The statistical relationship between artificially sweetened drinks and dementia disappears when the analysis controls for diabetes.
"This makes it more likely that there is a group of people who both use artificially sweetened drinks and are at higher risk of dementia, presumably because they have a risk factor, such as diabetes, for which a low sugar diet has been recommended.
"While the stroke effect remains even after diabetes has been taken into account, we should bear in mind that this is just one study with relatively small subgroups of participants."
Dr James Pickett, Head of Research at Alzheimer's Society, said: "This research does not show that artificially sweetened drinks cause dementia.
"But it does highlight a worrying association that requires further investigation."