A hazard ratio is the measure of how often an event occurs in one group compared to a control group over time. A hazard ratio of one (=1) indicates no difference in survival between the two groups, while a hazard ratio greater than one (>1) indicates that the treatment group had a shorter survival rate compared to the control group. 

Hazard Ratio Definition

A Swedish study of 233,879 males and 1,413,849 females without history of psychosis found that there was a 3.45 hazard ratio for first-onset schizophrenia in heavily smoking females and a 3.80 hazard ratio for heavily smoking males, compared to a 2.21 hazard ratio for lightly smoking females and 2.15 hazard ratio for lightly smoking males. 

University of Michigan - Ann Arbor

Several academic studies have corroborated the theory that there is a potential causal relationship between smoking and schizophrenia.

Causative Theory of Smoking and Schizophrenia


Ding JB, Hu K. Cigarette Smoking and Schizophrenia: Etiology, Clinical, Pharmacological, and Treatment Implications. Schizophr Res Treatment. 2021 Dec 13;2021:7698030. doi: 10.1155/2021/7698030. PMID: 34938579; PMCID: PMC8687814.



Cigarette Smoking and Schizophrenia: Etiology, Clinical, Pharmacological, and Treatment Implications


Scott JG, Matuschka L, Niemelä S, Miettunen J, Emmerson B, Mustonen A. Evidence of a Causal Relationship Between Smoking Tobacco and Schizophrenia Spectrum Disorders. Front Psychiatry. 2018 Nov 20;9:607. doi: 10.3389/fpsyt.2018.00607. PMID: 30515111; PMCID: PMC6255982.



Evidence of a Causal Relationship Between Smoking Tobacco and Schizophrenia Spectrum Disorders


In an Israeli study of 14,248 military recruits without any prior history of major mental illness, it was found that over a 4-16 year period, participants who smoked over 10 cigarettes a day were 2.28 times more likely to develop schizophrenia compared to their peers. 



Israeli Study of Military Recruits


Swedish Population Study


Another study also found that prenatal exposure to smoke also increased the risk of developing schizophrenia by 29%. 

Study of Prenatal Exposure to Smoke

A meta-analysis of research regarding the relationship between smoking and schizophrenia found that 6 out of 8 studies indicated a statistically significant association between tobacco smoking and onset of schizophrenic symptoms. The strength of the associations ranged from a 50% increase to a 600% increase. Two of the studies indicated that smoking 10+ cigarettes a day was associated with a hazard ratio over 3. 

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