Lithium is fairly toxic stuff. It is associated with thyroid dysfunction, blood dyscrasia, direct toxicity, and, over years, renal impairment.
But it is still used in bipolar disorder (BPD in the paper: most of the time I write BPAD — bipolar affective disorder, because BPD can also mean borderline personality disorder).
Despite the challenges using it. Because it is one of the few medications that have this kind of data: this is taken from a NHS patient dataset.

Of the 14,396 individuals with a diagnosis of BPD, 6671 were included in the cohort, with 2148 prescribed lithium, 1670 prescribed valproate, 1477 prescribed olanzapine, and 1376 prescribed quetiapine as maintenance mood stabilizer treatment. Self-harm rates were lower in patients prescribed lithium (205; 95% CI, 175-241 per 10,000 person-years at risk [PYAR]) compared with those prescribed valproate (392; 95% CI, 334-460 per 10,000 PYAR), olanzapine (409; 95% CI, 345-483 per 10,000 PYAR), or quetiapine (582; 95% CI, 489-692 per 10,000 PYAR).
… The suicide rate in the cohort was 14 (95% CI, 9-21) per 10,000 PYAR. Although this rate was lower in the lithium group than for other treatments, there were too few events to allow accurate estimates.

Here is the Kaplan-Meyer survival curve for first self harm attempt.

The article is open access: read the entire thing. Lithium probably saves lives. But it is out of patent, and some of the alternatives are promoted big pharma, which makes little to no money from generic treatments.