Table 2.

Risk bias assessment of the included studies using the Newcastle–Ottawa scale

Study	Selection				Comparability	Outcome
	Representation of exposed cohort	Selection of non-exposed cohort	Ascertainment of exposure	Change in outcome of interest	Comparability of cohorts	Assessment of outcome	Sufficient follow-up duration	Adequate follow-up cohorts	Risk of bias
Gaita (2007)11	*	*	*	*	*	*	*	*	Low
McCready (2011)12	*	*	*		*	*	*	*	Moderate
Hayashi (2014)13	*	*	*	*	*	*	*	*	Low
Müssigbrodt (2014)14	*	*	*	*	*	*	*	*	Low
Contreras-Valdes (2015)15	*	*	*			*	*	*	Moderate
Roh (2016)16	*	*	*	*	*	*	*	*	Low
Ikenaga (2017)17	*	*	*		*	*	*	*	Moderate
Chen (2018)18	*	*	*	*	*	*	*	*	Low

Asterisks indicate the star rating according to the Newcastle–Ottawa Scale. A maximum of one star can be given for each item within the Selection and Exposure categories. A maximum of two stars can be given for Comparability.