It is possible to identify drugs and poisons present in cadavers by analyzing blowfly larvae and pupae collected during forensic autopsies. The main purpose of this study was to use Lucilia sericata's larvae and pupae to identify drugs and poisons present in human cadavers. In an investigation, immature L. sericata fed meat treated with methamphetamine (MA) at various concentrations (45, 90, and 180 ng/mg) were analyzed to detect MA. Acetylation derivatization and liquid-liquid extraction (LLE) were used as sample preparation methods prior to gas chromatography-mass spectrometry (GC-MS) analytical instrumentation to find MA. According to this study, L. sericata can be used in toxicological testing to identify MA in a host body. All L. sericata larval stages, particularly the third stage larva, pupa and empty pupa tested were positive for MA. Larvae in their first instar produced weak peaks. The L. sericata post-feeding instar following the 45 ng/mg treatment showed the highest MA concentration. For the first time, derivatization using the acetylation approach was used to prepare samples, and successfully, excellent results were obtained. Low quantities of MA can be easily found in immature fly samples using GC-MS. It is important to analyze all samples including human tissues and insect samples, for postmortem drug testing. They can be utilized to find entire MA before they are excreted in excretory samples such as urine. Also, third instar larvae are a great and reliable sample for toxicological study.