CYP2C19 loss-of-function (LoF) carriers (any *2-*8 alleles) had significantly increased ischemic stroke risk (Hazard Ratio (HR) 1.53: 95% CIs 1.04 to 2.26, p = 0.031) and separately MI (HR 1.14: 1.04 to 1.26, p = 0.008) whilst on Clopidogrel, compared to non-LoF carriers in Cox’s proportional hazards regression models adjusted for age at first Clopidogrel prescription, sex, and the first 10 genetic principal components of ancestry. This evidence concerns the gene CYP2C19 and ischemic stroke.