Herbal medicines are usually marketed as natural products with multiple beneficial health claims. However, several studies have established that these products are usually adulterated with pharmaceuticals to increase their treatment effectiveness. Adulterating herbal products with pharmaceuticals can result in adverse herb–drug interactions, undermining their safety. Therefore, it is important to routinely analyse herbal products to determine if they have been adulterated with pharmaceuticals. A liquid chromatography coupled with tandem mass spectroscopy (LC–MS/MS) method was developed and validated for simultaneous identification and quantification of four commonly used antidiabetics—metformin (MET), gliclazide (GLZ), glibenclamide (GLC), and glimepiride (GLP)—in herbal medicines. Chromatographic separation of these analytes was done using a C-18 column and a gradient elution program of 0.1% formic acid in acetonitrile. The developed method showed detection limits and quantification limits ranging from 2.86 to 7.67 ng/mL and 8.64 to 23.24 ng/mL, respectively. The accuracy was above 80% for all analytes except MET (52%). This method was then applied to analyse 24 powdered herbal drugs sourced from the Kenyan market to check for adulteration. MET was detected in 17% of the samples, with concentrations ranging from 900 to 1969 ng/g. GLC, GLZ, and GLP were not detected in any sample. This method can therefore be used to analyse antidiabetic herbal medicines for adulteration using the four pharmaceuticals.