Researchers examined levels of lithium in drinking water and suicide rates in the prefecture of Oita, which has a population of more than one million.
The suicide rate was significantly lower in those areas with the highest levels of the element, they wrote in the British Journal of Psychiatry.
High doses of lithium are already used to treat serious mood disorders.
But the team from the universities of Oita and Hiroshima found that even relatively low levels appeared to have a positive impact of suicide rates.
Levels ranged from 0.7 to 59 micrograms per litre. The researchers speculated that while these levels were low, there may be a cumulative protective effect on the brain from years of drinking this tap water.
At least one previous study has suggested an association between lithium in tap water and suicide. That research on data collected from the 1980s also found a significantly lower rate of suicide in areas with relatively high lithium levels.
The Japanese researchers called for further research in other countries but they stopped short of any suggestion that lithium be added to drinking water.
The discussion around adding fluoride to water to protect dental health has proved controversial – criticised by some as mass involuntary medication.
In an accompanying editorial, Professor Allan Young of Vancouver’s Institute for Mental Health said “this intriguing data should provoke further research.
“Large-scale trials involving the addition of lithium to drinking water supplies may then be feasible, although this would undoubtedly be subject to considerable debate. Following up on these findings will not be straightforward or inexpensive, but the eventual benefits for community mental health may be considerable.”
Sophie Corlett, external relations director at mental health charity Mind said the research “certainly merits more investigation.
“We already know that lithium can act as a powerful mood stabiliser for people with bipolar disorder, and treating people with lithium is also associated with lower suicide rates.
“However, lithium also has significant and an unpleasant side effects in higher doses, and can be toxic. Any suggestion that it should be added, even in tiny amounts, to drinking water should be treated with caution and researched very thoroughly.”