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Background: Ratios of delta-9-tetrahydrocannabinol (THC) and cannabidiol (CBD) impact metabolism and therapeutic effects of cannabis. Currently, no states with legalized medical or recreational cannabis consider ratios THC:CBD in regulations. Objective: Determine what THC:CBD ratios are selected for use in clinical cannabis trials and what is the rationale. Methods: This is a systematic literature review of Central, CINAHL, Embase, PsycInfo, and PubMed of the last 10 years of English language medical cannabis publications highlighting THC:CBD ratios. Included were clinical studies of products containing and listing both THC and CBD ratios, percentages, or weighted amounts. Case reports and series, abstracts, reviews, and meta-analysis were excluded. Non-human, non-therapeutic, or studies examining approved cannabis pharmaceuticals were excluded. Results: Four hundred and seventy-nine (479) unique references were found, of which 11 met inclusion criteria. THC:CBD ratios listed and/or calculated: 1:0, 22:1, 2:1, 1:1, 1:2, 1:6, 1:9, 1:20, 1:33, 1:50, and 0:1. Rationale for ratios selected was often not listed, or simply trivialized as the ratios available to patients in the area, or ratios that were pharmaceutically available throughout the study country. One study compared ratios of high and low THC:CBD, but did not specify the ratios. Conclusion: The medical and scientific communities have not drawn substantive conclusions nor thoroughly explored THC:CBD ratios for "best practice" treatment of different disease processes and their sequelae. While there is evidence that cannabis provides medical benefits, research is lacking on standardization of medical cannabis use in modern medical practices.