Smoking and mental health - NCSCT

Smoking and mental health
evidence of associations between passive smoking and cognitive impairment. 173
This evidence is reviewed further in Chapter 4.
3.7 Future studies
Ultimately, observational data do not provide unequivocal evidence of causation,
and experimental studies to elicit further detail are either not possible in clinical
populations or unethical. Mendelian randomisation, 174 whereby genetic
information can be used to test causal hypotheses regarding the effects of
environmental exposures such as cigarette smoking, may allow a better
understanding of the causal relationship between smoking and mental health.
This requires specific polymorphisms that have been shown to be robustly
associated with measures of exposure (eg number of cigarettes per day). Given
the random assortment of genes from parents to offspring that occurs during
gamete formation and conception, genotype should not be related to potential
confounders (although confounding may still occur, eg due to linkage
disequilibrium with other loci, or pleiotropic effects whereby single genetic loci
are associated with multiple phenotypes). Therefore, a robust genetic influence
on smoking would be akin to a randomised trial where individuals are effectively
assigned to a high or low smoking exposure group, and could be used to test the
causal relationship between smoking and depression. A number of loci have
recently emerged that are convincingly associated with smoking. 175 These might,
in principle, be used to address questions of causation regarding the effects of
cigarette smoking. 101,176
3.8 Summary
> Although smoking, and high levels of dependence on smoking, are both
more common in people with mental disorders, the mechanisms underlying
these associations are uncertain.
> There is some evidence of common genetic determinants of both smoking
and specific mental disorders, particularly depression and schizophrenia.
> Experimental evidence suggests that nicotine can relieve symptoms of
anxiety, depression, schizophrenia and ADHD, although nicotine withdrawal
symptoms may then exacerbate symptoms of mental disorders.
> People with some mental disorders may use nicotine to ameliorate
symptoms such as depression or anxiety (the self-medication model).
> However, the symptoms of mental disorders can be confused with or
exacerbated by those of nicotine withdrawal, hence resulting in false
attribution of relief to effects on mental disorders.
> The effects of constituents of tobacco and tobacco smoke other than
nicotine on mood and cognition remain unclear.
52 © Royal College of Physicians 2013