Cancer

Unravelling the relationship between schizophrenia and lung cancer involves tackling the key confounding variable of smoking.^{[Bushe & Hodgson, 2010]} When attempts have been made to control for the levels of smoking observed among patients with schizophrenia, there appears to be a slightly reduced risk of lung cancer in this population.^{[Bushe & Hodgson, 2010]}

Other types of cancer are no less complicated, for example, as well as smoking, elevated glucose levels have been linked to some types of cancer, and schizophrenia has been shown to be a risk factor for diabetes.^{[Bushe & Hodgson, 2010; Correll, 2007]} Investigating these cancers in patients with schizophrenia involves attempting to control for the confounding factor of impaired glucose tolerance.^{[Bushe & Hodgson, 2010]} Moreover, there are likely to be risk variables that are simply not known, and so cannot be controlled in a study or a meta-analysis.^{[Bushe & Hodgson, 2010]}

It is clear that patients with schizophrenia have a higher risk of developing certain cancers, but it is not yet clear what this means.^{[Goldacre et al., 2005; Bushe & Hodgson, 2010]}

References:
Bushe CJ, Hodgson R. Schizophrenia and cancer: in 2010 do we understand the connection? Can J Psychiatry 2010; 55 (12): 761–767.

Correll CU. Balancing efficacy and safety in treatment with antipsychotics. CNS Spectrums 2007; 12 (10 Suppl 17): 12–20, 35.

Goldacre MJ, Kurina LM, Wotton CJ, et al. Schizophrenia and cancer: an epidemiological study. Br J Psychiatry 2005; 187: 334–338.