The Mahuang-Xingren (MX) herb pair, the combination of Herba Ephedrae (Mahuang in Chinese) and Semen Armeniacae Amarum (Xingren in Chinese), is a core component of traditional Chinese medicine formulations used to treat asthma and bronchitis. Although Xingren is considered to be toxic, MX is widely used in the clinic and has few adverse effects. The mechanism underlying detoxification of Xingren by Mahuang in MX remains unknown and merits investigation. To determine the mechanism underlying detoxification of Xingren by Mahuang in MX. Acute toxic effects were evaluated in mice after oral administration of Mahuang, Xingren, and MX aqueous extracts. Synergism, additivity, and antagonism were quantified by determining the CI (combination index) and DRI (dose-reduction index), which were calculated by the median effect method. High performance liquid chromatography analysis of bioactive compounds (ephedrine, pseudoephedrine and amygdalin) in aqueous extracts and data from previous pharmacokinetic studies in rats were combined to explore the potential mechanism of toxicity antagonism by the components of MX. Moreover, the cytotoxic effects of amygdalin and amygdalin activated by β-glucosidase (including different proportions of l-amygdalin and d-amygdalin) were also investigated. Mahuang prevented and antagonized the acute toxicity of Xingren and allowed escalation of the Xingren dose. Pearson correlation analysis indicated that the proportion of d-amygdalin was closely correlated with the antagonism of Xingren toxicity. The antagonism of its acute toxicity was primarily attributed to stereoselective metabolism of amygdalin. Interestingly, the process was facilitated by Mahuang, which led to reduced levels of the d-prunasin in vivo and thus reduced toxicity. Furthermore, the mechanism was also evaluated by testing the cytotoxicity of amygdalin. Metabolism of d-amygdalin was a major cause of cytotoxicity and no stereoselective metabolism occurred in culture medium. A comprehensive study of Xingren detoxification in the context of the MX combination suggested that stereoselective metabolism of amygdalin facilitated by Mahuang may be the crucial mechanism underlying detoxification of Xingren in the MX combination. Therefore, Mahuang acts to enhance and control the effects of Xingren in the MX combination. These results illustrate the rationale behind the combination of Mahuang and Xingren. Copyright © 2015 Elsevier Ireland Ltd. All rights reserved.