Hydrochlorothiazide is a diuretic used to treat hypertension that belongs to class IV of the Biopharmaceutics Classification System. The drug was evaluated by quality control, thermal characterization tests, and pharmaceutical formulation compatibility studies. It was concluded that the generic drug, Lab 2, was not a pharmaceutical equivalent. The compounded drugs, Lab 5 and Lab 6, produced unsatisfactory but expected results, since there is no requirement for dissolution and dissolution profile testing for the commercialization of these products. In a compatibility study, lactose and mannitol were shown to be incompatible with HCTZ, which may explain the lack of equivalence of the generic pharmaceutical product, associated with other situations.