Monthly vitamin D supplements won't increase life expectancy for older Australians, a study has found.

However, results from the QIMR Berghofer Medical Research Institute's landmark trial indicates large monthly vitamin D doses may slightly increase the risk of dying from cancer.

In one of the country's largest clinical trials, more than 21,000 Australians aged between 60 and 79 took vitamin D every month for up to five years as part of the study.

Lead researcher Professor Rachel Neale said the D-Health Trial was launched in 2014 amid uncertainty over vitamin D's health benefits.

While Prof Neale said monthly supplements would be beneficial to people who were vitamin D deficient, the study's results indicated it did not reduce the mortality rate in Australians aged over 60.

"Most Australians are not vitamin D deficient according to current guidelines and the D-Health Trial suggests that if people are not deficient, taking vitamin D does not increase the chance of living for longer," she said in a statement.

In the study, some participants were given vitamin D while others received a placebo and the mortality rate was compared between the groups.

Prof Neale said exploratory results indicated taking a large amount of vitamin D monthly may slightly increase the risk of dying from cancer but added that "the difference (between groups) was small and this increased risk is unproven".

"Out of an abundance of caution we would advise that if people want to take a vitamin D supplement, they take a regular daily dose rather than a larger monthly dose," she said.

"This study does not support increasing the amount of vitamin D people are advised to have in their blood."

The research has been published in The Lancet Diabetes & Endocrinology journal.