A systematic review of longitudinal studies examined the association between cigarette smoking and depression and anxiety. The review included 148 studies, categorizing outcomes into smoking onset, smoking status, smoking heaviness, tobacco dependence, and smoking trajectory. Results showed mixed evidence, with some studies finding positive associations in both directions (smoking leading to mental health issues and vice versa), while others found no association. Nearly half of the studies reported that baseline depression or anxiety was linked to later smoking behavior, while over a third found evidence that smoking was associated with later depression or anxiety. However, few studies supported a bidirectional relationship, and very few reported null results. The review highlights the complexity of the relationship between smoking and mental health, with potential bidirectional causality, shared risk factors, or confounding variables. While longitudinal studies provide valuable insights, they cannot alone establish causality. The review suggests that future studies should employ methods like Mendelian randomization (MR) to better understand causal relationships. The findings indicate that the association between smoking and mental health is not consistent across different populations, genders, ethnicities, or clinical statuses. Overall, the evidence is inconclusive, and further research is needed to clarify the direction and nature of the relationship between smoking and mental health.A systematic review of longitudinal studies examined the association between cigarette smoking and depression and anxiety. The review included 148 studies, categorizing outcomes into smoking onset, smoking status, smoking heaviness, tobacco dependence, and smoking trajectory. Results showed mixed evidence, with some studies finding positive associations in both directions (smoking leading to mental health issues and vice versa), while others found no association. Nearly half of the studies reported that baseline depression or anxiety was linked to later smoking behavior, while over a third found evidence that smoking was associated with later depression or anxiety. However, few studies supported a bidirectional relationship, and very few reported null results. The review highlights the complexity of the relationship between smoking and mental health, with potential bidirectional causality, shared risk factors, or confounding variables. While longitudinal studies provide valuable insights, they cannot alone establish causality. The review suggests that future studies should employ methods like Mendelian randomization (MR) to better understand causal relationships. The findings indicate that the association between smoking and mental health is not consistent across different populations, genders, ethnicities, or clinical statuses. Overall, the evidence is inconclusive, and further research is needed to clarify the direction and nature of the relationship between smoking and mental health.