Table 2.

NOS scores for the included studies.

Study	Selection	Comparability	Outcome	NOS Score
Case–control studies
Gao et al. [9]	***	*	***	7
Rakašević et al. [19]	***	**	***	8
Neilands et al. [20]	***	*	***	7
Soriano-Lerma et al. [21]	***	*	**	6
Arısan et al. [22]	***	**	***	8
Tada et al. [23]	***	**	***	8
Isehed et al. [24]	***	**	**	7
Carcuac et al. [25]	***	**	**	7
Pranno et al. [26]	***	*	***	7
Wawrzyk et al. [27]	**	*	***	6
Ciobanu et al. [28]	*	*	***	5
Al-Ahmad et al. [29]	**	*	***	6
Zheng et al. [30]	**	*	***	6
Korsch et al. [32]	**	*	***	6
Persson et al. [12]	***	*	***	7
da Silva et al. [34]	***	*	***	7
Tamura et al. [35]	***	*	***	7
Shi et al. [36]	***	*	***	7
Sanz-Martin et al. [37]	***	*	**	6
Yeh et al. [39]	**	*		3
Maruyama et al. [40]	***	*	***	7
Kröger et al. [41]	**	*	*	4
Cohort studies
Wang et al. [31]	***	*	***	7
Ghensi et al. [33]	****	*	***	8
Yu et al. [38]	****	*	***	8

NOS Scale star system for quality assessment of studies.The higher number of stars (*), the better quality. The maximum (*) possible are 9: **** for Selection, ** for Comparability and *** for Outcome.