Katuk (Breynia androgyna (L.) Chakrab. & N.P.Balakr) is a local plant with various medical and food benefits. This plant contains various secondary metabolites, including tannins, polyphenols, alkaloids, saponins, and flavonoids. A toxicity test is intended to determine the safe doses of katuk leaves. This study aimed to conduct acute toxicity testing of katuk leaf extract in mice. The toxicity level was observed from the LD50 and observation of potential effects after administering the test doses. The toxicity testing was conducted using the Thompson and Weil method. Swiss webster White mice were divided into 5 groups of 5 mice each. The control group was given 1% NaCMC solution, while the other four groups (treatment groups) were given different doses (50, 300, 2000 mg/kg BW) of katuk leaf extract. The results showed that the LD50, calculated using the Thompson and Weil method, was 9954.05 mg/kg BW. The study concludes that the katuk leaf extract is nontoxic and safe in experimental animals at the doses given. However, some behavioural changes (stress and diarrhoea) were observed 24 hours after the treatment at 300 and 2000 mg/kg BW doses.