Is nicotine harmful?

Could you elaborate on the potential risks associated with nicotine consumption? Are there any scientific studies that conclusively show nicotine to be harmful to human health? What are the specific health conditions that have been linked to nicotine use? How does nicotine affect the body and what are the long-term consequences of regular nicotine exposure? Are there any SAFE levels of nicotine consumption, or is it best to avoid it altogether?