This certainly does seem like a step above anything that's been done before. Fluoride is intended to be basically inert as far as ingestion goes--- its purpose is to improve tooth strength, but not to have any medically noticeable internal effects when the water's drunk. Adding a substance that is medically active when internally ingested as a default drinking-water option would be quite a step. At the very least, we might want to get a lot better handle on what the possible downsides are. There are these studies showing a good preventative association with suicides, but does increasing lithium intake do anything negative? It would be surprising if it had no possible negative effects, so the question is more what they are and how likely/serious they are.
