There is no good evidence that a nutrient drink being sold online in the UK to "help" people with early Alzheimer's actually slows the disease, say experts.
Latest trial results in patients who took Souvenaid did not find it preserves memory and thinking.
The authors say in Lancet Neurology that bigger studies are needed to show if the product can work as hoped.
And consumers should be aware that the £3.49 per bottle drink "is not a cure".
Manufacturer Nutricia says its drink should only be taken under the direction of a doctor, specialist nurse or pharmacist.
What is the drink?
Souvenaid comes in strawberry or vanilla flavour and contains a combination of fatty acids, vitamins and other nutrients.
Taken once daily, the idea is that the boost of nutrients it provides will help keep Alzheimer's at bay in people with the earliest signs of this type of dementia.
But the latest phase two clinical trial results do not prove this.
What the trial found
The study involved 311 patients with very early Alzheimer's or mild cognitive impairment. All of them were asked to take a daily drink, but only half were given Souvenaid - the other half received one with no added nutrients.
After two years of participating, the patients were reassessed to see if there was any difference between the two groups in terms of dementia progression, measured by various memory and cognitive tests.
The treatment did not appear to offer an advantage, although patients in the Souvenaid group did have slightly less brain shrinkage on scans, which the researchers say is promising because shrinkage in brain regions controlling memory is seen with worsening dementia.
But experts remain cautious.
Prof Tara Spires-Jones, a dementia expert at the University of Edinburgh, said: "Some of the other tests of brain structure and function were promising, but overall this study indicates that a specific change in nutrition is unlikely to make a large difference to people with Alzheimer's, even in the early stages.
"There is strong evidence that a healthy lifestyle including exercise and a healthy diet can help reduce risk for developing dementia, but once the brain damage starts, a dietary intervention is unlikely to stop the disease."
Another expert, Dr Elizabeth Coulthard from University of Bristol, said people should think carefully before buying something that is, as yet, unproven.
Dr David Reynolds, from Alzheimer's Research UK, advised: "If people are worried about their memory, or are considering buying and taking Souvenaid as a supplement to manage their diet, then it is important that they discuss this with their GP."
A spokeswoman from Nutricia said: "We are pleased that this adds to the body of evidence for Souvenaid and we remain committed to ongoing and further clinical research."