The Republic of Moldova ranks among the countries with relatively poor water resources. This is the reason why their protection and adequate utilization continue to be a problem at the national level. One of the factors causing the deterioration of their quality is the increase in the volume of untreated wastewater. Pharmaceutical compounds released into the aquatic environment are famed for their persistence in the environment. That is why studies of their influence on aquatic ecosystems have prevailed recently. One of the drugs often used in the treatment of tuberculosis, which is a fairly common disease in the Republic of Moldova, is isoniazid (INH) [1,2]. The presence of INH in aquatic systems is inevitable, so the goal of this research was to find out how this drug mixture affects the natural waters' selfpurification capacity. For this purpose, the following systems were modeled: INH-H2O-H2O2PNDMA-hν; INH-H2O-H2O2-Cu(II)-PNDMA-hν. All model systems were irradiated with the polychromatic lamp DRT-400. In this way, the inhibition capacity and stationary concentration of OH radicals in the modeled systems were determined, using the radical trap PNDMA (p-nitroso-N,N-dimethylaniline) [3]. From the obtained results, it was found that increasing the concentration of INH in the system leads to an increase in the inhibition capacity and a decrease in the OH radical concentration. The values for the inhibition capacity when INH was added at concentrations ranging from 3.8·10-5 M to 3.2·10-4 M were found to be on the order of 106 to 107 s-1. According to the inhibition capacity parameter, the systems are classified as highly polluted waters. Self-purification processes decrease considerably in water systems with an increase in INH concentration. It was discovered that when the concentration of hydrogen peroxide in the system increased, the concentration of OH radicals increased and the inhibition capacity decreased. This proves that isoniazid effectively scavenges OH radicals, which are generated during hydrogen peroxide photodissociation. It was found that when Cu(II) ions are added to the system, the concentration of OH radicals rises 3–6 times and the inhibition capacity falls. This is explained by the fact that Cu(II) ions catalyze the photodissociation of hydrogen peroxide, producing a much greater quantity of OH radicals. So, in the presence of Cu(II) ions, according to the inhibition capacity parameter, the systems are classified as polluted waters. Based on this, it was shown that INH, which has a reducing character, consumes the oxidative equivalents in aquatic systems. Thus, INH leads to a reduction in the self-purification capacity of aquatic systems.