Research on the therapeutic effects of nicotine is a very young field of knowledge because so far the focus has been on its negative impact on the body.

An excerpt from the article 15 facts about nicotine

Psychiatrists have long wondered why almost 90 percent of patients with schizophrenia and depression smoke cigarettes heavily. They take into account that this is some form of self-medication, because these patients struggle with constant anxiety, irritability, and the inability to focus on one thing, and nicotine can alleviate these symptoms. But it's also possible that nicotine causes them, as smoking has been proven to increase the risk of depression, suicide, and panic attacks.