Well, I was sleepy a lot for starters. It was nothing for me to always want afternoon naps, and to sleep in to 10 am or later. Always felt tired.
My blood pressure has gone way down since I discontinued use. When I look it up, this is listed as one of the side effects. Another, (and Jeff can confirm this) was edema in my legs, ankles, and feet. They were always swollen with fluid. I'm convinced the extra fluid may have been a contributing factor to the recent infection in my toe that turned into a blood infection, and a week in the hospital.
Lastly, and I never knew until I went to quit, but it's addictive. I went through terrible withdrawals when I was in the hospital and didn't have access to it. Reminded of the opioid withdrawals I went through before I kicked that. Thought because kratom was natural, I'd be ok.
@CigarStone had a side effect from it to that cost him a doctors visit and having to take a steroid for a spell. That prompted him to quit it was well.