The stuff used in Captain Phillips was khat - a stimulant that has widespread use in the Horn of Africa (Somalia, Ethiopia, etc). Unlike kratom it is illegal in the US on the federal level.
Kratom does have a mildly stimulating side, but feels much more like an opiate. Again, it does work on the mu opioid receptor in the brain - so it has effects, addictive potential, and withdrawal potential similar to traditional opiates. On top of that there are alkaloids that haven't been studied in this plant, so we really don't know how else it works on the brain.
There are a lot of fanboys and sellers online working overtime to promote this stuff as completely harmless. Again - there are many people who can use it responsibly. But it is what it is, you need to be very cautious. And the highly concentrated tinctures that I was using are just bad news all around.
Stay safe everyone and thanks for the welcomes.