To weigh in with a random lurkers two cents, i used to use it as a pre-practice supplement for college football. I had basic (not extract) powder, and i would put two or three grams in a bottle of hot water and drink that right before practice. It tasted like a really shitty green tea, but i would basically chug it and it was no big deal (no nausea or anything)
There were multiple reasons for this:
1. It was not detected by the drug tests (at the time, i have no idea if this has changed)
2. As many have mentioned, a low dose like this was arguably imperceptible in terms of the effects you receive. It was not like i was suddenly high as f*ck on an opiate. However, in a low dose, opiates act like stimulants, and when you are running around in football practice that low dose effect is magnified and basically gave me a constant high energy stream, that left me feeling happy and energized.
When I took it like this, the sled or shoots became a fun activity that i enjoyed, instead of something to be feared.
In my case, it was highly effective. It made going to football practice enjoyable and greatly improved my playing ability, if not actually making me better player, but by greatly increasing my mood and dulling my feelings of fatigue. Mentally, i felt way better, which translated onto the field.
Suffice to say, after a season of this, i realized that the reason why i was playing football was because i was high, and so I quit football and the use of kratom.
I have not noticed any long term effects.