There were some limitations in this meta-analysis as follows. First, wherever possible, we used the full adjusted forms of RR and OR controlling for factors such as age, gender, race, mental disorder, drug abuse, smoking, marital status, body mass index, educational level, employment status, income, and living alone. However, the confounding effect was not completely ruled out because some studies reported crude forms of RR or OR estimates. This issue may lead to overestimation of the overall measures of association. Second, there were 12 studies (mostly old studies) that seemed potentially eligible for inclusion in this meta-analysis, but their full texts were not accessible. We requested the relevant institutes to find the full texts for us, but they could not. We tried to contact the corresponding authors to send us the full texts, but the authors did not respond. This issue may raise the possibility of selection bias.
