Chemically, cannabinoids are not really antidepressants (which mostly affect the serotonin system) or pain killers (opioids affect the opioid receptors). Cannabinoids have a variety of effects due to the endocannabinoid system, which is naturally occurring in the human physiology and does affect mood, memory, etc. However, it's simply not chemically accurate to call them pain killers or antidepressants, cannabis lies in a separate category.