This observational study was published in the Journal of Cannabis Research in December 2020, and showed a 95% effectiveness rate.
A decrease in symptom intensity levels was reported in 95.51% of Cannabis usage sessions, an increase in 2.32% of sessions, and no change in 2.16% of sessions.
"Mid to higher THC levels are statistically significant predictors of increased symptom relief, while CBD levels and inhalation method (joint, pipe, vape) are largely not."
"Our findings suggest that self-directed use of Cannabis flower, especially that with higher THC levels, is associated with significant improvements in at least short-term feelings of distress in many users, likely a contributing factor to its widespread popularity and consumption in the U.S."
Link to the Journal Paper is here.