There have been many documented cases of individuals dying from heroin withdrawal. This is especially true for addicts recently incarcerated, or others undergoing non-medically supervised detoxification.
Fentanyl is a synthetically produced opioid prescribed to control pain during surgery, treat severe post-surgical pain, and to treat chronic pain in those for whom other opioids are no longer effective. It is a drug that can be used relatively safely and effectively when used as prescribed, but one that can be deadly when misused or used illegally.
Kratom is an unregulated herb that delivers effects similar to those of opioids and stimulants. Because of its classification as an herb it is widely available in the U.S. However, there are serious concerns about the potential for kratom to lead to dependence or addiction.