The GWAS results for OUD thus far have revealed a small number of significant loci that have not been replicated across the different studies. The lack of consistency may be driven by the relatively small sample sizes, particularly if OUD is highly polygenic and many variants have small effect sizes, as has been observed in other psychiatric disorders like schizophrenia [43], Genetic or environmental variation between the study populations, even genetically similar ones such as Americans and Australians of European descent, might also explain the variable results.
