Opioids are commonly used as analgesics to relieve chronic pain and have high abuse potential. Due to their strong potency and trace concentration in plasma, a robust analytical method is necessary for quantification in forensic and pharmacology fields. Hence, this study developed and validated a simple, rapid, and robust method for the simultaneous determination of 12 opioids and metabolites which were available legally by prescription or abused for non-medical purposes, in plasma samples by simple liquid extraction and high-performance liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS). We compared the extraction recovery of our sample pre-treatment to two other sample pre-treatments (namely QuEChERS and simplified QuEChERS) and showed that the method used in our study gave the highest recoveries. The method validation followed the European Medicines Agency guidelines, including selectivity, carryover, accuracy and precision, dilution integrity, matrix effect and freeze/thaw stability. This method's accuracy ranged from 85% to 115% with a precision less than 15%, within the acceptable range of the validation protocol. The lower limit of quantification of the method ranged between 0.05 μg L and 0.38 μg L among 12 opioids/metabolites. Stability was assessed, with all opioids observed as relatively stable at 0.5 μg L and 5 μg L levels under -20 °C and 25 °C storage conditions. In summary, the developed method has the potential to achieve simultaneous analysis for monitoring opioids in forensic and pain management regimens using a simple sample pre-treatment.