I checked this thread for an expansion of that initialism, which I didn't recognize. It likely stands for "low-dose naltrexone." https://ldnresearchtrust.org/what-is-low-dose-naltrexone-ldn

"a prescription medication used primarily to treat alcohol use disorder and opioid use disorder" (in addition to the off-label uses discussed here)

Also a LDN user, that drug saved my life. I bought it from a shady dude online, the initial effect was so strong that I thought they shipped me meth instead - that wore off after a week but the lifting of the brain fog persisted.