Overdoses are rare but can, like anything else, occur. The liver in particular doesn't appreciate very large doses of phenibut. The body tolerates 10g pretty easily. Fatal overdoses could occur but probably require insane doses. I take 4g/night and have for 6 years, and though I was surpised, my liver and kidney function (per blood tests and urinalysis that I orderded) are absolutely normal (aside for blood albumin I'm the higher end of the spectrum). Will probably get an MRI soon but I'm assuming it normal. I expected elevated enzymes or perhaps a little protein in the urine, but neither occcured (which is interesting considering I drink a fair amount of alcohol night). Phenibut at these doses doesn't seem to have a significant dose on organ function. But the sample size of this study is pretty low (where n=1).
Still, avoid phenibut. I never was a fan of phenibut, I only started after quiting 900mg/day Lyrica habit. I was fine other than having severe insomnia for two months straight. I gave up and started taking phenibut. I've been too busy from work and life to quit phenibut since (despite still sleeping pretty shittily)