A simple and accrued spectrophotometric method was developed to estimate Doxycycline as pure and in its formulation. The method included using a novel reagent neutral red dye the present method. The method was based on the oxidation of Doxycycline with an excess amount of N-bromosucinimide in an acid medium, then the residual amount of N-bromosucinimide oxidized the neutral red dye and caused bleaching of the color of the dye. The absorbance of the dye's color did not bleach measured at wavelength 525 nm. The absorbance is proportional to the concentration of Doxycycline in the solution. The linearity was from 1 to 15 µg/ ml, and the higher concentration above 15 µg/ ml gave deviation from Beer's law. The important analytical parameter included molar absorptivity and Sandell's sensitivity index, which were calculated and equal to 3.67 x104 l/mol.cm, 0.01261µg/cm2, respectively. An application part included estimating Doxycycline in its dosage form (capsule) from different manufacturers with satisfactory results. Keywords: Doxycycline, Neutral red, Bleaching Color, Oxidation,

A simple spectrophotometric method for the determination of doxycycline in aqueous medium using oxidative coupling reaction has been described. The method was based on the reaction of doxycycline with 4-aminoantipyrene (4-AAP) which gives soluble and stable red colored product that gives the highest absorption at the wavelength 515 nm, and the linearity of Beer's law was in the concentration range from 5 to 110 µg/ml, and the value of the determination coefficient for the standard curve was 0.9995, which statistically indicates that it has excellent linear characteristics. The molar absorptivity was calculated and found to be 1.387x10 3 l.mol -1 .cm -1 . Sandell's sensitivity value, the Limit of detection (LOD), and ‫‬ limit of quantitation (LOQ) were calculated and equal to 0.3334 μg/cm 2 , 0.114 and 0.38 µg/ml respectively the method was successfully applied for the determination of doxycycline in its pharmaceutical preparation.