This study was published in the Harm Reduction Journal in July 2021. It focused on learning more about patients that substituted prescription drugs with cannabis, the type of drugs substituted, and the effect on prescription drug use.
The prescription medications substituted were pain medications (67%), antidepressants (24%), and arthritis medications (20%). Of the 2,841 adults that used cannabis with a specific medical purpose, "38.1% reported termination of prescription drug use, and 45.9% a substantial decrease in prescription drug use." That means that over 84% were successful in reducing or eliminating prescription drugs by using cannabis.
Over 65% of respondents that substituted cannabis for prescription drugs used CBD oil while 36.6% used hash or cannabis flower. Almost 66% found cannabis MUCH more effective than prescription drugs, and 85% said the side effects of prescription drugs were MUCH worse than with cannabis.
The full paper is here.