The steep rise in the number of people suffering opioid addiction has helped spawn the widespread use of another substance: kratom, a green powdered herbal supplement that is widely available and virtually unregulated.

Derived from the leaves of a tree native to Southeast Asia and sold in the United States online and in bodegas and head shops, kratom has long been used as a mood booster, energy supplement and pain reliever.

It is also increasingly being used by those who swear by it as a curb for opioid addiction. Some veterans also say it helps control symptoms of post-traumatic stress. Several million Americans are now believed to use kratom.

One is Andrew Turner, whose PTSD, herniated discs and movement problems affecting his face and neck were so severe after multiple deployments with the Navy that he took as many as 20 prescription medications, including opioids, daily. “I was on the path to suicide, and losing hope,” Mr. Turner said.