Table 1.

Qualitative analysis of articles included in the systematic review.

Author/Year	Study Design	N_Control Group	HN_Control Group	IC_Control Group
Michaud et al., 2008 [21]	Cohort	48,275	0.89	0.71–1.10
Huang et al., 2019 [23]	Cohort	38,092	2.11	1.63–2.73
Michaud et al., 2016 [22]	Cohort	19,933	1.17	0.94–1.47
Lee et al., 2017 [24]	Cohort	1235	1.5	1.05–2.10
Wu et al., 2019 [25]	Case-control	2171	1.81	0.76–4.34
Hujoel et al., 2003 [27]	Case-control	5240	0.49	0.19–1.26
Boland et al., 2013 [26]	Case-control	1240	1.5	1.05–2.10