[Confusion]

In practice there usually isn't a problem when making these sorts of decisions, because the decision requires popular support. If adding lithium reduces suicides by a factor of three, probably adds to the general, measureable, happiness of many more and is shown not to have any adverse 'side effects', including effects like 'being less critical when appraising opinions of authority figures' (which is a sort of effect that likely would be investigated, as a natural part of investigating the mind altering qualities of adding lithium to the water) then almost everyone would decide in favor of adding the substance.