Opioids may be the most difficult drugs to withdraw from. Heroin, fentanyl, carfentanil, OxyContin, Vicodin, methadone, and Tramadol; these are just a few of the many opioids that are commonly abused in the United States. Drugs like heroin, fentanyl, and carfentanil are often the strongest and most addictive of these drugs, and therefore usually have the most intense withdrawal symptoms.