abstract: Background: Hash oil, a cannabis preparation that contains ultra-high concentrations of tetrahydrocannabinol (THC), is quickly gaining popularity in the United States. Some evidence suggests that hash oil might produce greater intoxication and more severe negative effects than marijuana. This study examined whether the subjective effects of hash oil are more extreme than the subjective effects of marijuana and whether frequency of hash oil use is associated with the subjective effects of marijuana and hash oil. Method: Past-year cannabis users (n = 1,268) were recruited online to complete a questionnaire about the subjective effects of cannabis. Participants who reported past-year use of both hash oil and marijuana (n = 574) rated subjective effects of each type of cannabis in the following positive and negative domains: positive affect, cognitive enhancement, negative affect, cognitive impairment, physiological effects, reduced consciousness, and psychotic-like experiences. Results: Results of within-person comparisons showed that hash oil was rated as producing lesser positive effects (Hash oil: M = 4.53, Marijuana: M = 5.55, t = 14.67, p < .001) than marijuana. Negative effects of hash oil were minimal for the full sample (n = 574) and for both frequent and infrequent hash oil users. In general, the frequency of hash oil use was not associated with the subjective effects of marijuana but more frequent hash oil use was associated with rating hash oil as producing greater positive effects ( = 0.28, t = 6.86, p < .001) and lesser negative effects ( = -0.16, t = -3.83, p < .001). Findings were unchanged after controlling for sex, medical cannabis use, and frequency of marijuana use. Conclusions: Hash oil produced lesser positive effects than marijuana. Negative effects of hash oil were minimal, suggesting that extreme negative effects may be unlikely for experienced cannabis users. / Dissertation/Thesis / Masters Thesis Psychology 2019