Gene editing therapies are designed to minimise off-target editing. However, it is not widespread practice for common polymorphisms to be considered when identifying potential off-target sites . Nevertheless, genetic variants should be included as they have the potential to alter existing, or to generate new, off-target sites. To facilitate the consideration of common polymorphisms when designing targeted gene therapies we developed PopOff, a web-based tool that integrates minor allele frequencies from the gnomAD variant database into an off-target analysis. We used PopOff to analyse predicted off-target loci from guide RNAs used in four clinical trials and thirty-four research publications. From an analysis of sixty guides, we identified that approximately 20% of off-target loci overlap with a common polymorphism. Of these sites, 6.93% contained variants that reduce the level of mismatch between the off-target locus and guide, and therefore may increase off-target cleavage. In addition, we identified that 0.34% of common polymorphisms generated novel PAM sites, resulting in off-target loci that standard workflows would miss. Our findings demonstrate that common polymorphisms should be considered when designing guides to maximise the safety of CRISPR-based gene therapies. However, this may be problematic in populations where the breadth of genetic diversity remains uncharacterised.