Background: a comparative analysis of safety and determination of the mean lethal dose (LD50) of doxycycline hydrochloride and glucosamine combined preparation “Doxycycline with glucosamine” in two ratios 1:1 and 1:2 in order to select the most appropriate one for further study. Material and methods: The acute toxicity of doxycycline hydrochloride, glucosamine hydrochloride and combinations on their base were investigated in the experiment in different ratios (1:1) and (1:2) by the method of V. B. Prozorovskiy. There were used 36 white rats. They were divided into 6 groups of 6 animals each. Animals of test groups received doxycycline doses ranging from 500 mg/kg to 5000 mg/kg. The drug was administered intraperitoneally in appropriate doses, dissolving it in the necessary amount of physiological saline solution. For the calculation of the median LD50 the percentage of mortality in each group was determined after 14 days. Results: The study found that LD50 of doxycycline at single oral administration in rats is 1893.03 ± 286.2 mg/kg. Glucosamine hydrochloride showed LD50 > 15000 mg/kg. The combination of doxycycline with glucosamine (1:1) demonstrated LD50 – 5197.8 ± 434.3 mg/kg. The combination of doxycycline with glucosamine (1:2) had LD50 > 10000 mg/kg. Conclusions: Glucosamine reduces the toxic effect of doxycycline in combination from the low toxic class to the class of practically non-toxic substances. It expands the range of its therapeutic effect and increases safety. It allows considering the combination of 1:2 as the promising object for further studies of its specific activity.