Two-sample Mendelian randomization120,121 was used to evaluate the potential causal association between TUD and genetically correlated traits using samples of European ancestry only (without UKBB). Of the 76 traits that showed significant genetic correlations (Supplementary Table 31), we removed 45 that were phenotypically similar (e.g., BMI and obesity). From each category, we selected those traits with higher rg. Therefore, we tested 31 traits for a causal relationship with TUD. We inferred causality bidirectionally using three methods: weighted median, inverse-variance weighted (IVW) and MR-Egger, followed by a pleiotropy test using the MR Egger intercept.122,123 Instrumental variants were those associated with the exposure after clumping (r2 = 0.01) and at p<1.0E-05. We considered causal effects as those for which at least two MR tests were significant after Bonferroni correction (p = 0.05/31 = 1.61E-03) and that showed no evidence of violation of the horizontal pleiotropy test (MR-Egger intercept p>0.05).
