Lithium, normally in the form of lithium carbonate (Li2CO3) is widely recognised as the 'gold standard' prescribed drug for treating bipolar disorder, depression and mania.
In widespread use since the early 1970s, lithium carbonate was the first FDA approved drug for 'mood control'.
However, despite more than forty years of intense research, and millions of treated patients,
“The precise mechanism of action of Li+ as a mood-stabilizing agent is currently unknown.” Source: Wikipedia
Further, for reasons that are also unknown, therapeutic lithium has little or no effect on some individuals :
“Lithium is generally regarded as the first-line agent for the management of BP, although it does not work for everyone. For instance, Geddes and colleagues performed a meta-analysis of five randomized controlled trials comparing prophylactic lithium therapy with placebo in BP and found that lithium is more effective than placebo in preventing recurrence of illness, with 60% in the lithium group remaining well over 1–2 years compared with 40% in the placebo group.”
source: External Predictors of Lithium Response in Bipolar Disorder