Methadone opioid drug, molecular model. Methadone is a synthetic type of opioid, a class of drugs that are powerful pain relievers (analgesics). It mimics the effects of drugs like heroin and so helps to combat withdrawal symptoms. When a drug addict is treated with methadone the dose must gradually be reduced with time as methadone itself is highly addictive. Methadone's chemical formula is C21H27NO2. Atoms are represented as spheres and are colour-coded: carbon (C, grey), hydrogen (H, turquoise), nitrogen (N, blue) and oxygen (O, red).