My friend has a medical marijuana license + eats "edibles" every now + then. i read that thc is harmful. could it cause him irreversible memory loss?

Only if a teenager: Marijuana has 2 main active agents, thc ; cbd. It is proven to reduce inflammation, help muscle spasms ; nausea ; can modulate immune function- see http://bit.Ly/1bypqfd heavy use by those 17 ; under can cause long-term memory issues. It does not seem to cause long-term cognitive impairment in adults: http://ti.Me/1cddscc cbd counteracts the memory issues related to thc: http://bit.Ly/1d0kkdi.