In order to evaluate the clinical safety of a modification of the classical formula Si Jun Zi Tang (Wei Qi Booster® (WQB)), acute and subchronic mouse oral toxicity tests were conducted. An acute oral 7-day mouse study using 80 Kunming mice randomly divided into saline control and 7 dosages of WQB (range 2.0 -15.06 g/kg BW) was conducted to determine LD50. No mortality or adverse effects were observed in any parameter tested which included food consumption, clinical toxicity, body weights, organ/body weight ratios (liver and kidney) and histologic morphology of the liver, spleen, lung, kidney and duodenum. To determine the maximum tolerated dose of WQB, 30.12 g/ kg BW was given twice daily for 7 days. No mortality or adverse effects were observed in any parameter tested in this study which included all parameters evaluated in the LD50 study along with hematology. In the subchronic oral study, 15 mice were divided into 3 groups with the control group fed normal chow diet and each of the other two groups either receiving normal chow with 1% WQB or levamisole hydrochloride 0.01 g/kg BW (positive control) for 90 days. No mortality or adverse effects were observed in the 1% WQB in any parameter tested which included all parameters evaluated in the acute studies and with clinical chemistry replacing hematology. Results of these preclinical safety studies conducted with WQB at the highest dose tested which is 30.12 g/kg BW for 7 days and 1% of food consumed for 90 days demonstrated no toxicity.