To estimate whether paired drugs were interconvertible by a feasible chemical reaction or required a more esoteric transmutation, we subjected the drug pair reaction SMILES to the Transformer-based RXNMapper model, which returns an atom-to-atom comparison illustrating the structural relationships within pairs, as well as an atom-mapping confidence score. Atom-mapping confidence scores were determined for the forward and backward reactions and converted to atom-mapping confidence distances (amcd), defined here as one minus the confidence score. In most cases the amcd values were similar for forward and backward reactions, however since the difference was sometimes substantial (Figure ), we used the mean amcd of forward and backward reactions for our analysis. The mean amcd value spanned the entire range between low and high distance (last entry, Figure /3a/4a) except for the PMB set, which mainly contains high confidence distances as the structures are too big for the model to map with high confidence. Further, the amcd was not correlated with any of the selected molecular similarities (last entry, Figure ).