Disorders such as Depersonalization and Derealization occur because of damage done to the brain oftentimes by a traumatic event. While there are some prescription drugs out there that aid with these disorders, a lot of them can also make things worse. It truly depends on the individual and their brain chemistry.
Below is a link to a PDF file that contains a list of herbs that can be used to help treat damaged areas of the brain. While some may help, it could have negative effects for others. I in no way promote simply trying these herbs without heavy research firsthand.
Valerian is one specific plant that I use to help ease anxiety and reducing my HPPD and Visual Snow. It has also worked wonders for me to get a night of great sleep with no hangover feeling the morning after.
Link to Herb PDF Page
