There is a strong association between smoking and mental health disorders. For example, it has been observed that people who smoke are two and four times more likely to have depression and schizophrenia, respectively, compared to those who do not smoke. However, we currently have a poor understanding of the reasons for this observed association; for example, it is uncertain whether the observation is because smoking directly increases an individual's risk of developing a mental health disorder, or whether it is because developing a mental health disorder makes a person more likely to start smoking.

The researchers used an "event history framework", as this method has two key advantages. Firstly, it takes into account temporal differences between smoking and mental health (e.g., most people who start smoking do so before the age of 20, whereas mental health disorders are typically diagnosed between the ages of 30 and 60). Secondly, the event history framework allows for the fact that both smoking and mental health disorders have a substantial genetic component (there are genes that influence whether an individual is likely to start smoking, and similarly genes that influence how likely they are to develop disorders such as depression and schizophrenia).

The team of researchers analyzed data from UK Biobank, a population database containing genotypic and phenotypic information from 500,000 individuals. Their analyses identified a strong causal relationship between smoking and mental health disorders. Specifically, they estimated that when an individual starts smoking, their risk of developing depression, bipolar or depression increases by 250%, but that if that individual subsequently quits smoking, their risk reduces to only 50% higher than those who have never smoked.

'- It is well-known that smoking has a negative impact on people's health', explains Doug Speed. He continues, '- However, I believe many people would be surprised to find out that the dangers of smoking relate  o mental health, as well as physical health. Our study not only quantifies the benefits to society of discouraging individuals from starting smoking, but also of encouraging current smokers to quit.'

You can read full details of the study in Acta Psychiatrica Scandinavica: (https://onlinelibrary.wiley.com/doi/full/10.1111/acps.13601)

Contact: Professor Doug Speed, Center for Quantitative Genetics and Genomics