By Emma Wilkinson
BBC News health reporter
Prostate cancer is a major killer
Vitamin D is an effective treatment for prostate cancer in some patients, a UK study suggests.
A once daily dose reduced PSA level - an indicator of severity of disease - by as much as half in 20% of patients.
There has been much interest in vitamin D in prostate cancer after studies linking risk of the disease to sunlight exposure, the researchers said.
One expert agreed the findings were encouraging but said it needed to be tested in a bigger population.
The trial - results of which are due to be published in the journal BJU International - was set up after one patient got better when his wife bought him some vitamin D tablets.
Professor Jonathan Waxman, said the example had prompted him to assess the effects in a wider group of patients.
Out of 26 men with recurrent prostate cancer, who took a daily dose of vitamin D2 bought from the chemist, five responded to the treatment.
In two the PSA level, fell by more than half, in two by 25-50% and in one man it fell by less than 25%.
The effects in one man were sustained for 36 months.
Study leader Professor Jonathan Waxman, from Imperial College London, said vitamin D therapy was effective and well-tolerated.
"It's very interesting - there has been no significant trial of vitamin D.
"This is a treatment which is unlikely to have significant toxicity and is a welcome addition to the therapeutic options for patients with prostate cancer."
He agreed that a further trial in a larger number of patients, comparing vitamin D with a dummy pill was warranted.
One theory is that vitamin D interferes with the effect of the androgen receptor, which is stimulated by hormones such as testosterone and implicated in prostate cancer.
John Neate, chief executive of The Prostate Cancer Charity, said it was not the first study looking at vitamin D in the disease but a consensus on the benefits had not been reached.
"This small scale study investigating the use of vitamin D as a 'stand alone' treatment for men with progressive prostate cancer provides a valuable additional perspective.
"Many men with prostate cancer may wonder whether they should take vitamin D supplements to control their disease.
"This study does not answer that question, but maintaining a good level of vitamin D is recommended as part of a generally healthy lifestyle.
"The role of supplements in the prevention and treatment of prostate cancer is an area which deserves a greater level of research attention."
Professor Malcolm Mason, Cancer Research UK prostate cancer expert based at Cardiff University agreed the results were encouraging but more evidence was needed.
"We advise men with prostate cancer to consult their doctor before taking vitamin D supplements."