Abstract Animal models are often used to test the safety and efficacy of drugs in cell culture and body systems. Several researchers deliver drugs to rodents in drinking water, although it has some limitations. For instance, drug stability, water consumption, and body mass fluctuation may change drug dose. Thus, we investigated telmisartan (TEL) stability in mice drinking water by UV spectrophotometry, and if water intake and body mass were fluctuated then it changes the predicted drug dose. The results showed that UV spectrophotometry could detect TEL at the wavelength of 300 nm, and the concentration curve was set between 1.25 and 60 µg/mL. Also, it remained stable in mice drinking water for 7 days at the predicted concentration. Mice gained weight after 8 weeks on a high‐fat high‐sucrose diet, and it was reduced by TEL 5 mg/kg/day after 3 weeks. Although water intake remained stable, not adjusting the TEL concentration weekly by body mass would lead to higher consumption of TEL by mice. In conclusion, we demonstrated that body mass and water intake fluctuations significantly change the amount of drug that the animal receives, which would add bias to the experiment. TEL remains stable for at least 7 days in wrapped mice water bottles in the animal care facility, and UV spectrophotometry proved to be a simple and low‐cost method to detect TEL in mice drinking water.