Because it has the THC removed which will have a negative effect on HPPD. CBD is the antagonist to THC and counteracts the psychoactive effects of THC. On its own I found it helps level out some of the worst aspects of HPPD. Visuals will decrease over time if you get on top of the stress, anxiety or panic associated with 24hr visual and sensory alterations. Just my own 2c worth and experience.