GSTM1 and headache disorder: Moreover, polymorphisms in genes encoding for enzymes involved in phase II metabolism, such as in UDP-glucanosyltransferases [e.g., UGT2B7*2 (NSAIDs)] and GSTM1 (GSTM1 *0/*0, CBZ, valproate sodium), as well as SNPs in drug transporters such as MDR1 (ABCB1, opioids, topiramate) organic cation transporter 1 (SLC22A1, opioids) or MRP2 (ABCC2, CBZ), have been associated with modulations of drug safety in headache disorders.