Nicotine is the substance in the tobacco leaf that makes you want to smoke again. It reaches the brain quickly and triggers a release of dopamine: that is where the short-lived sense of relief and focus comes from, and why it asks to be repeated.
Addiction is a real part of the problem. But most of the harm from smoking comes not from nicotine but from the products of combustion: tar, carbon monoxide and other substances that reach the lungs with the smoke (US FDA). These are linked to most cases of cancer, lung disease and heart disease.
Nicotine is not found only in tobacco. In very small amounts it occurs in vegetables of the nightshade family — aubergine, tomatoes, potatoes. One gram of aubergine holds about 100 nanograms of nicotine; matching the dose of a single nicotine patch would take 70 to 210 kg of aubergine a day (Domino et al., 1993). The fact explains where nicotine comes from — and says nothing about its safety.
Nicotine is addictive in any form. So the question is not whether nicotine is harmful, but what else enters the body along with it. Stopping completely remains the lowest-risk option.
A useful question for tonight. What else besides nicotine do I breathe in with the smoke?
When to get help. If you smoke and want to stop, a doctor can offer evidence-based approaches that improve your chances. For urgent symptoms call 103 or 112.