The growth of the human population from year to year causes an increase in the use of pharmaceuticals in everyday life, which has an extremely unfavorable effect on the environment, primarily the water environment, causing its pollution. In addition to the use of pharmaceuticals, the use of plastic, which is ubiquitous in our lives, is also increasing. Degradation of plastic creates microplastics, which have a large specific surface to which pollutants in the environment bind. The interaction of pollutants with microplastics can increase its toxic effect on the environment and human health. The aim of this work is to investigate the adsorption of the pharmaceutical carbamazepine (CBZ) on microplastic particles prepared from unaged poly(ethylene terephthalate) (PET 0) particles and samples subjected to accelerated aging under UV radiation for 56 days (PET 56) and their adverse effect on the aquatic environment. Before determining the toxicity of microplastics on which a pharmaceutical is adsorbed, it is important to determine the toxicity of individual components. Their harmful effect was evaluated by performing standard acute toxicity tests on the green microalgae Pseudokirchneriella subcapitata (ISO 8692). The results showed that UV aging increases the proportion of smaller fractions of microplastic particles, which indicates an increase in the amorphous phase, the structure becomes less ordered, the mechanical properties are lost, thus the hardness and stiffness of the polymer decrease. The results of carbamazepine adsorption correspond to the Freundlich model, which indicates adsorption in multiple layers. Adsorption is most pronounced at lower temperature, in combination with PET 0, which indicates physisorption. In the case of toxic evaluation, the pristine material showed higher toxicity compared to the material aged for 56 days under UV radiation, and the combination of CBZ + CaCl2 + PET 0 showed the highest toxicity, while the individual components showed a lower toxic effect.