This in vitro study evaluated the therapeutic equivalence of two multisource (generic) formulations of 100-mg phenytoin as immediate-release tablets available in the Peruvian pharmaceutical market, compared with the reference medicine, to establish interchangeability. The mean weight, hardness, and content of active substance were evaluated, prior to analyzing the dissolution profile. USP dissolution apparatus 2 (paddle) was used at 75 rpm with 900 mL of dissolution medium at 37 ± 0.5 °C at pH levels of 1.2, 4.5, and 6.8. The generic and reference formulations had similar weight or drug content, but hardness values were significantly different (p = 0.029). At pH 1.2, the generic products were considered therapeutically equivalent to the reference product based on similarity factor (f2) and dissolution efficiency values; however, at pH 4.5 and 6.8, there were differences in dissolution performance based on f2 values below the acceptable range.