Zuotais regarded as the king of Tibetan medicine. However, the major starting material ofZuotais mercury, which is one very toxic heavy metal. This has aroused serious doubts on the biosafety ofZuotacontaining drugs. In this study, we quantified the Hg contents in fourZuotasamples, monitored the release of Hg in simulated gastric/intestinal juice and evaluated their cytotoxicity to Caco-2 cells. Our results showed that the Hg contents inZuotasamples were in the range of 566–676 mg/g. Fortunately, the release of Hg fromZuotasamples was very low in simulated gastric juice, and much lower in simulated intestinal juice. Direct contact ofZuotawith Caco-2 cells led to dose-dependent cytotoxicity, including activity loss and membrane leakage. The toxicity was closely related to apoptosis, because the caspase 3/7 levels of Caco-2 cells increased after the exposure toZuota. Interestingly,Zuotasamples inhibited the oxidative stress at low concentrations, but the toxicity could be relived by antioxidants. The possible toxicity should be attributed to the cellular uptake ofZuotaparticulates. Beyond the cytotoxicity, significant differences amongZuotasamples from different institutions were observed, suggesting that the preparation process ofZuotahad meaningful influence of its biosafety. The implications to the safety and clinical applications ofZuotaare discussed. [ABSTRACT FROM AUTHOR]