Aims To perform a meta-analysis of the association between CYP2C19 loss- and gain-of-function variants and cardiovascular outcomes and bleeding in patients with coronary artery disease treated with clopidogrel, and to explore the causes of heterogeneity between studies.

Methods A comprehensive literature search was conducted. A random-effects model was used to summarise the results. In the presence of between-study heterogeneity, a meta-regression analysis was performed to identify study characteristics explaining this heterogeneity.

Results Patients who carried a loss-of-function allele, mainly CYP2C19*2, did not present an increased risk of a cardiovascular event, HR =1.23 (95% CI 0.97 to 1.55). Substantial heterogeneity was observed between studies (I² =35.6), which was partially explained by the study sample size: the pooled HR was higher among studies with a sample size <500 patients (HR =3.55; 95% CI 1.66 to 7.56) and lower among studies with a sample size ≥500 (HR =1.06; 95% CI 0.89 to 1.26). CYP2C19*2 was associated with an increased risk of a stent thrombosis (HR =2.24; 95% CI 1.52 to 3.30). The gain-of-function allele, mainly CYP2C19*17, was associated with a lower risk of cardiovascular events (HR =0.75; 95% CI 0.66 to 0.87) and a higher risk of major bleeding (HR =1.26; 95% CI 1.05 to 1.50).

Conclusions Not only CYP2C19 loss-of-function but also gain-of-function alleles should be considered to define the pharmacogenetic response to clopidogrel. The results question the relevance of the CYP2C19 loss-of-function alleles in the prediction of major cardiovascular events beyond stent thrombosis in coronary patients treated with clopidogrel. The gain-of-function variant is associated with a lower risk of cardiovascular events but a higher risk of bleeding.