Opium derivatives as the addictive substances cause many medical and social problems. In treatment of opiates addiction except psychotherapy a long time pharmacotherapy with methadone or buprenorphine are apply. In a recent study both moderate- and high-dose methadone treatment resulted in reductions in illicit opioid use. The aim of the study was develop fast high sensitive liquid chromatography coupled with mass spectrometry for determination of methadone and EDDP after microwave assisted extraction. Developed method for methadone and EDDP determination were linear over the range 5 to 1000 ng/ml for methadone and its metabolite. The method exhibit a good intra and inter day precision, the microwave assisted extraction was also satisfactory with recovery superior to 80%. The mean concentration of methadone in plasma of patients receiving 40 mg of drug per day was 194 ng/ml but in second group (receiving 90 mg of methadone/day) 263 ng/ml. The ratio of the doses was 2.25 but for methadone concentration only 1.36. The much higher ratio (3.25) of concentrations was noticed for EDDP. This observation can support opinion other authoress about polymorphism of methadone pharmacokinetics and pharmacodynamics.