GSTM1 and lip and oral cavity carcinoma: The meta‐analysis of four included studies of tissue samples showed a significantly (p < .001) increased GPx activity in the oral cancer group, when compared to the control group, whereas three included studies of erythrocyte samples displayed a significantly (p < .001) decreased GPx activity in the oral cancer group than the control group with the pooled standardized mean difference value of −2.766 moles/min/g Hb at 95% CI (−3.297 to −2.234).