What is, overall, safer for your body: alcohol or marijuana?

My girlfriend tells me she uses marijuana to calm herself if she is having a stressful day. She has been doing this for many years. She says smoking pot is like self-medicating — it is better than using anti-depressants. She also claims smoking pot helps with depression because of how it helps produce serotonin in the body. What is serotonin and how does it help depression? Does smoking pot really help with managing your anxiety, depression, etc.?

I want to know what it is when you spontaneously trip without dropping tabs or anything. I've never had any drugs except marijuana. It is weird. I know that tripping is an induced psychosis, so am I psychotic or what? I was seeing a shrink and will be going back into treatment next semester. I think substance use is boring and a complete copout on life. I've made a lot of discoveries without needing to trip. Why does this happen? Tell me what you know.