There is no cure for Alzheimer's disease
A drug once used to treat hayfever "significantly improves" symptoms in patients with mild to moderate Alzheimer's disease, research suggests.
Dimebon was once licensed in Russia as an antihistamine but was taken off the market when better drugs came along.
Now US researchers have found it can improve memory, behaviour and ability to conduct simple activities like eating in patients with dementia.
Experts were cautiously optimistic about The Lancet study findings.
In the trial of 183 people who all had untreated mild to moderate dementia, which was carried out in Russia, half were given 20 mg of dimebon 20 three times a day while the rest were given a dummy pill.
After six months, all were given tests such as memorising a list of words and performing simple tasks.
Those taking the drug scored four points lower on a scale designed to measure severity of Alzheimer's disease - meaning they were less badly affected.
Patients taking the drug scored better than they did at the start of the study but patients taking the placebo got worse over the six-month period.
In a smaller group of patients who continued with the trial for a further six months there was an even greater seven-point gap between those on dimebon and those on placebo.
It is not clear exactly how the drug works but it has been shown in animals to have a protective effect on nerve cells in the brain.
Study leader Dr Rachelle Doody, from Baylor College of Medicine in Houston, Texas, said the ongoing improvement seen in the study was particularly important.
"At present no approved therapies for mild to moderate Alzheimer's disease have shown increasing improvement over 12 months."
Alzheimer's drugs already on the market are restricted for use in the NHS in those with moderately severe disease.
Rebecca Wood, chief executive of the Alzheimer's Research Trust, said: "The recent announcement of an additional phase III trial is encouraging.
"More research on this scale is desperately needed if we are to offer hope to the 700,000 people in the UK who live with Alzheimer's and other dementias."
The Alzheimer's Society agreed that more research was needed.
"These initial findings imply that dimebon could be more effective than treatments currently licensed for people with Alzheimer's, however this was a modest sized study."
Professor Robin Jacoby, an expert in psychiatry at the University of Oxford said there was a lot of interest in "old" drugs which had been used for other conditions as they would cost less to develop.
"On the face of it, these are fairly remarkable results.
"I would simply say that these things need to be treated with a great deal of caution and we want to see the results replicated."
A separate study by UK researchers also published in The Lancet reported that trials of vaccine developed to prevent progression of Alzheimer's disease have not been as successful as hoped.
Although the vaccine cleared the characteristic build up of clumps of protein in the brain of Alzheimer's patients it did not prevent neurological decline.