An important feature of living organisms, acquired in the process of evolution, is the ability to adapt to various external influences and changes in the environment, to maintain homeostasis and regulatory processes in organs, systems and the body as a whole. However, with modern breeding technologies, the physiological capabilities of the animal's body are often unable to change as quickly as required by the conditions of industrial maintenance, which leads to stress. Vitamin preparations and biologically active feed additives are widely used by veterinary medicine specialists to increase resistance to stressful situations, disease prevention, increase resistance, safety and productivity of farm animals. Preparation L-tsyn, solution for injection (manufactured by BIOTESTLAB LLC), is a drug, the action of which is due to the prescription combination of active substances, namely: butaphosphane, B vitamins (vitamin B12, vitamin B3) and L-carnitine. The aim of the study was to investigate the effectiveness of the drug L-tsyn on horses under the influence of stressors of various natures. Two experiments on working horses with constant, excessive physical activity with chronic exercises and with complex therapy of intoxication with signs of acute renal failure in horse were conducted. Clinical trials of the drug were performed on working horses aged 5–15 years, of different sexes. The drug was administered in a dose of 20–25 ml per animal, 1 time per day for 5 consecutive days. The clinical condition of the experimental animals was observed for 21 days from the start of the drug. The morpho-functional state of the animal's body before and after the drug was established according to the indicators of the clinical condition, morphological and biochemical parameters of the blood, which were determined according to generally accepted methods. In working horses under excessive load the drug L-tsyn activated the processes of hematopoiesis, improved the physiological state of the body and contributed to the increase the general tone of the body. The drug as part of complex therapy in the treatment of intoxication, showed hepato- and nephroprotective properties, reduced the manifestations of intoxication, helped to restoration of the animal’s normal physiological state. According to the results of clinical, hematological and biochemical studies, it can be concluded that the drug L-tsyn has a stimulating effect on metabolic processes in the animals’ body, shows tonic properties, increases the body's resistance to toxins and adverse environmental factors and can be used as monotherapy and for the complex treatment of diseases of various etiologies in horses.