In recent decades, the development of pharmaceuticals and their use in medicine have significantly improved quality of human life. However, as a result of their production and use, they end up in the environment, where they pose a risk to the organisms living in it. Pharmaceuticals, their metabolites and degradation products can be very persistent and accumulate in the environment, which can lead to high concentrations that have a toxic effect on the ecosystem. For this reason, it is necessary to investigate their stability and persistence in the environment as well as the possible degradation products that are formed. In this work, the stability and behavior of two pharmaceuticals, atazanavir and nirmatrelvir, were tested by forced degradation experiments. Forced degradation was performed with a 0.003 mM solution of atazanavir and a 0.1 mM solution of nirmatrelvir under the following conditions: alkaline hydrolysis in the presence of a 1.0 M NaOH solution at room temperature for a period of 24 hours, acid hydrolysis in the presence of a 1.0 M HCl solution at room temperature over a period of 24 hours, oxidation in the presence of a 30% H2O2 solution at room temperature over a period of 5 hours, thermal decomposition at 30 °C and 40 °C over a period of 7 days, neutral hydrolysis at room temperature (25 °C) over a period of 7 days and photolytic decomposition under UV light at 600 W/m^2 at 25 °C over a period of 6 hours. HPLC methods were developed and validated for analysis of atazanavir and nirmatrelvir. The developed methods were found to fulfill all validation criteria: linearity with a coefficient of determination above 0.9900, accuracy with recoveries between 70 and 130% and precision with RSD values of less than 10%. The results of the forced degradation tests showed the expected stability of the tested compounds under most of the applied conditions.