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 ten 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."