Methadone. Hand holding a beaker containing a solution of the drug methadone. Methadone is opiate analgesic that can be substituted for opioid drugs such as heroin (diamorphine). The methadone drug 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.