Flutamide (FLUT) is an antitumor drug of importance capable of existing and making in solution and gas phase in two tautomeric types via prototrophic tautomerism. The two tautomer were examined here with a computational procedure. The calculation of relative energy levels at the B3LYP/6-311G++(d,p), 6-311++g, and 6-311++g(d) levels of theory suggest that T2 tautomer is the one with the utmost stability in the gas phase, which corresponds to NBO, dipole momentum, and thermochemistry parameters. The observations are helpful to better understand proton transfer tautomerization procedures. The findings are also discussed in the framework of chemoinformatics rules to handle tautomerism so that the findings of tautomer types can be helpful for a better action of chemical compounds in biologic milieus.