Major Meta-Study on the Health Effects of Cannabis

Summary of findings, with a lot of the references and numbers removed:

Results: 101 meta-analyses were included. From RCTs supported by high to moderate certainty, cannabis based medicines increased adverse events related to the central nervous system, psychological effects, and vision in people with mixed conditions (GRADE=high), improved nausea/vomit, pain, spasticity, but increased psychiatric, gastrointestinal adverse events, and somnolence among others. (GRADE=moderate).

Cannabidiol improved 50% reduction of seizures and seizure events (GRADE=high), but increased pneumonia, gastrointestinal adverse events, and somnolence (GRADE=moderate).

For chronic pain, cannabis based medicines or cannabinoids reduced pain by 30% (0.59 (0.37 to 0.93), GRADE=high), across different conditions (n=7), but increased psychological distress.

For epilepsy, cannabidiol increased risk of diarrhoea (2.25 (1.33 to 3.81)), had no effect on sleep disruption (GRADE=high), reduced seizures across different populations and measures (n=7), improved global impression (n=2), quality of life, and increased risk of somnolence (GRADE=moderate).

In the general population, cannabis worsened positive psychotic symptoms and total psychiatric symptoms (GRADE=high), negative psychotic symptoms, and cognition (GRADE=moderate).

In healthy people, cannabinoids improved pain threshold, unpleasantness (GRADE=high). For inflammatory bowel disease, cannabinoids improved quality of life (GRADE=high).

For multiple sclerosis, cannabinoids improved spasticity, pain, but increased risk of dizziness, dry mouth, nausea, somnolence (GRADE=moderate).

For cancer, cannabinoids improved sleep disruption, but had gastrointestinal adverse events (GRADE=moderate).

Cannabis based medicines, cannabis, and cannabinoids resulted in poor tolerability across various conditions (GRADE=moderate).

Evidence was convincing from observational studies (main and sensitivity analyses) in pregnant women, small for gestational age, low birth weight; in drivers, car crash; and in the general population, psychosis. 

Harmful effects were noted for additional neonatal outcomes, outcomes related to car crash, outcomes in the general population including psychotic symptoms, suicide attempt, depression, and mania, and impaired cognition in healthy cannabis users (all suggestive to highly suggestive).

Conclusions Convincing or converging evidence supports avoidance of cannabis during adolescence and early adulthood, in people prone to or with mental health disorders, in pregnancy and before and while driving. Cannabidiol is effective in people with epilepsy. Cannabis based medicines are effective in people with multiple sclerosis, chronic pain, inflammatory bowel disease, and in palliative medicine but not without adverse events.

Other than the rather remarkable effect on seizures and a modest effect on nausea (n.b., we have much better anti-nausea drugs), I think pretty much all the positive findings here can be explained by the simple fact that people like being high. This study also adds to the data showing that long-term cannabis use is dangerous for your mental health, although possibly not any more dangerous than whiskey.