Lithium levels in tap water and psychotic experiences in adolescents

Partner just told me a story she used to put Lithium drops in her friend water (reasoning that after 10 days when it comes to action people usually come to their sense and seek professional help, but it didn't work in this case and friend tried to self healing and eventually relapsed).

"Aquatic lithium has no significant effect on incidence of self-reported psychosis."

As I understand it, there is supposed to be about a factor of two difference between the dose that actually does any good, and acute toxicity. So trace amounts are not expected to help those people who could benefit from lithium medication.

Please correct me if I have misunderstood this.