Developing a simple and fast method to analyze possibly adulterated synthetic drugs in suspected herbal medicines (HM) is both methodologically and commercially significant. This paper constructs a new approach named local straight-line screening (LSLS), to the solution of the problem, after carefully observing the characteristics of the spectral line shapes. LSLS can be applied to both the qualitative and quantitative analysis of suspected HM, based solely on infrared spectrum of the possible synthetic drug and the suspected HM without any sample pretreatment. The concept and the application of the method are exemplified by analysis of sibutramine hydrochloride (SH), an anti-obesity medicine, adulterated in some commercially available HM-based diet pills, and compared with MSFDSS, and SNICA methods. The results show that, despite highly overlapping spectra and unresolved data, it is possible with LSLS to obtain an accurate discrimination and determination of SH in the HM-based diet pills. This allows the method to be potentially used in the primary screening of other synthetic drugs suspiciously adulterated in HM, with high rapidity, accuracy and cost-effectiveness.