Experts are questioning whether diet drinks could raise depression risk, after a large study has found a link.
The US research in more than 250,000 people found depression was more common among frequent consumers of artificially sweetened beverages.
The work, which will be presented at the American Academy of Neurology's annual meeting, did not look at the cause for this link.
Drinking coffee was linked with a lower risk of depression.
People who drank four cups a day were 10% less likely to be diagnosed with depression during the 10-year study period than those who drank no coffee.
But those who drank four cans or glasses of diet fizzy drinks or artificially sweetened juice a day increased their risk of depression by about a third.
Lead researcher Dr Honglei Chen, of the National Institutes of Health in North Carolina, said: "Our research suggests that cutting out or down on sweetened diet drinks or replacing them with unsweetened coffee may naturally help lower your depression risk."
But he said more studies were needed to explore this.
There are many other factors that may be involved.
And the findings - in people in their 50s, 60s, 70s and 80s and living in the US - might not apply to other populations.
The safety of sweeteners, like aspartame, has been extensively tested by scientists and is assured by regulators.
Gaynor Bussell, of the British Dietetic Association, said: "Sweeteners used to be called 'artificial' sweeteners and unfortunately the term 'artificial' has evoked suspicion. As a result, sweeteners have been very widely tested and reviewed for safety and the ones on the market have an excellent safety track record.
"However, the studies on them continue and this one has thrown up a possibly link - not a cause and effect - with depression."
She said the study was a "one-off" and did not mean that sweeteners caused depression.
"For a start, people who suffer from depression may latch on to the idea that it is their sweetened beverages that caused it and so add a bias to their reporting of past intake, especially as 'soda' in the US is demonised even more than in the UK. Also, it may be that drinking 'diet' drinks is a marker for obesity or diabetes which in themselves can cause depression.
"Non-calorific sweeteners can play a useful role in the diets of those trying to lose weight and diabetics and it is certainly not advocated that people should replace their diet sodas with more coffee."
Beth Murphy, at the mental health charity Mind, said: "We would urge anyone who is affected by depression to follow the advice of their GP or other medical professional in regards to their treatment."