Next time you're filling up your gas tank, check out the convenience store offerings. You'll see flashing lights advertising kratom and, if you head inside, you'll see displays of powders, capsules, extracts and gels. Sometimes kept behind a locked glass case, sometimes kept behind the counter, kratom is available to anyone over the age of 18, much like cigarettes. Kratom is not sold in big-box stores, but it is a billion-dollar industry in the United States.