Which of the following is the main problem associated with using lithium to treat mood disorders?

A) Lithium increases the likelihood of completed suicidality by 40 percent.
B) The difference between an effective dose of lithium and a toxic dose is small.
C) It has proved ineffective when used in combinations with vitamins and dietary supplements.
D) It has caused mental impairment in over 20 percent of the people who have taken it.

The main problem associated with using lithium to treat mood disorders is that the difference between an effective dose and a toxic dose is small, which can lead to severe side effects and toxicity if not carefully monitored. The other answer choices are not accurate. Lithium has been shown to decrease the risk of completed suicide, it is generally well-tolerated and does not cause mental impairment in the majority of patients, and its efficacy has not been shown to be affected by vitamin or dietary supplement use.