No other compound in Tibetan medical pharmacology seems to be as fascinating, controversial, and enigmatic as tsotel (btso thal, lit. 'cooked ash'), the processed mercury sulphide ash that provides the base material of many of the popular Tibetan 'precious pills' (rin chen ril bu). The compound contains--apart from numerous herbs and other ingredients--eight metals and eight rock components. Tsotel practices, which can be traced back to the thirteenth century in Tibet, are considered the pinnacle of Tibetan pharmacology. The commercial value of tsotel gives it a strong economic and social life of its own. This paper analyses the social life of tsotel from an anthropological perspective and sketches key aspects of tsotel's biography, which in one way or the other are linked to medical, political, and religious perceptions of mercury: tsotel events with their political and institutional agendas; the value of tsotel as a medical, religious, and political commodity; safety and toxicity debates; and tsotel's religious and political efficacy. I argue that the social life of tsotel is increasingly linked to perceptions of toxicity and safety because of its chief ingredient, mercury, being contested in a globalised arena of tightened international regulations as well as the recent attention given to heavy metal toxicity issues in Asian medicines. Also, several fundamental misconceptions of the substance of mercury itself, its processed form of mercury sulphide, and of the contamination of herbal ingredients with heavy metals will be highlighted. Examples are based on ethnographic fieldwork with Tibetan medical practitioners and pharmacologists in India and Nepal.