You know, I've always read that long term kratom users (in Thailand) develop darkening of facial skin pigment with heavy use. I've been using kratom in 30 - 50g doses per day consistently for like 4ish years (cant remember exactly when I started). I have not experienced any pigment darkening or skin issues of any kind. I really haven't had any extreme negative responses, other than typical side effects. Just my experience.
Speaking of "Black Face", Turbo Negro is the shit!
As an anecdote, as I've posted a lot recently, I've been using kratom since ~ 2014. I started using it to replace opioids from pain management; but I was not physiologically dependent on opioids at the time (Rx was PRN). I have been heavily dependent on heroin, morphine, methadone, hydro/oxy, codeine etc many times like 10 years prior, just not when I made the switch to kratom.
I sure as hell hope skin issues don't develop. I also worry about testosterone and thyroid issues, though haven't experienced them at all (30 y/o Male). Had routine medical blood work done 4 months ago with primary care provider and all was wnl.
Though, I personally would choose methadone or buprenorphine maintenance therapy over kratom use at this point; I posted my reasoning earlier in this thread. I know that is not a popular opinion, but that is how I feel. I am really physiologically dependent on kratom. Serious and heavy withdrawl develops within 24 hours of discontinuation. Having gone through a myriad of opioid withdrawal situations in the past (as stated above), I can say for me, the discontinuation syndrome from kratom is similar to natural and synthetic opioids.