In the subgroup with PDW (Q2) and the MTHFR 677 TT genotype, folic acid treatment had a 2.31-fold increased risk of stroke (HR: 2.31; 95% CI: 1.06–5.04; p = 0.035), while folic acid treatment within the PDW (Q4) and the MTHFR 677 TT genotype subgroup reduced stroke risk by 59% (HR: 0.41; 95% CI: 0.20–0.85; p = 0.016). The gene discussed is MTHFR; the disease is Stroke.