Smoking is addictive due to nicotine, which conditions consumption and leads to physical and psychological dependence in smokers. Smoking causes respiratory, cardiovascular, and cancer diseases that are harmful to both smokers and those exposed to secondhand smoke. While nicotine has some antidepressant and anxiety-relieving effects, it is not used pharmaceutically due to the discovery of more effective and non-addictive antidepressants in the 20th century.