It's extremely hard to find anything on Google because such FACTS don't serve the corporate oligarchy. Here is one source but there are others if you dig deep enough and follow all the bread crumbs which are usually buried a dozen pages deep on Google and obscured in bio-chemical tech speak.
Here's a quote of the relevant portion.
New research suggests that cigarettes have antidepressant qualities. Among the thousands of chemicals found in a combusted cigarette, monoamine oxidase inhibitors (MAOI) have been detected. Monoamine oxidase is involved in removing the neurotransmitters norepinephrine, serotonin, and dopamine from the brain. MAOI’s stop the monoamine oxidase from extracting these neurotransmitters. For these reasons, MAOI’s were used as one of the first antidepressants in the early days of psychiatric medication as we know it. Since then, MAOI’s are only used as a last resort, because of their debilitating side-effects and withdrawal symptoms. MAOI side effects include: nausea, diarrhea, headache, drowsiness, insomnia, skin reactions, dizziness, muscle spasms, inorgasmia, stunted sex drive, weight gain, muscle aches, and increased suicide risk. MAOI withdrawal symptoms include: agitation, irritability, pressured speech, insomnia, nightmares, hallucinations, disorientation, paranoia, aggressiveness, catatonia, and electric shock feelings. In essence, when you try to quit smoking, not only are you withdrawing from nicotine alone. Along with the additives, tar, and carcinogens, you are withdrawing from a dangerous, outdated antidepressant.