There is increasing use of head-to-head clinical trials in dermatology when establishing the efficacy of a new treatment. Active comparator trials (ACTs) can be classified into three distinct study trial designs: non-inferiority, equivalence and superiority. A better understanding of the statistical parameters, such as acceptable treatment differences (also known as the margin or delta), is necessary to properly design and interpret findings of active comparator trials (ACTs) in the field of dermatology. Therefore, the objective of this study was to summarize the maximum acceptable treatment differences in clinical trials that examine the efficacy of an oral or biologic psoriasis therapy with an active comparator. We conducted a systematic search using MEDLINE, Scopus, EMBASE, Cochrane Central Register of Controlled Trials, LILACS, Web of Science and ClinicalTrials.gov from inception to 31 August 2017. All ACTs with adult participants that had a primary outcome of the Psoriasis Area and Severity Index score were included. Bibliographies of articles were further reviewed. Two investigators independently assessed for article inclusion and separately completed data extraction of predefined data points. When there was a disagreement, a third investigator was consulted. Of the 49 ACTs included, there were 13 superiority, eight non-inferiority and seven equivalence trials. Another 21 studies had inadequate information for classification. All of the non-inferiority trials reported the margin, one of the superiority and six of the equivalence trials stated the treatment difference explicitly. For superiority trials, acceptable treatment differences ranged from 14% to 20%. The non-inferiority studies reported lower bound margins ranging from −20% to −10%. The equivalence trials reported upper and lower bound margins ranging from ±12.5% to ±18%. The results demonstrate the need for harmonization in the conduct of dermatological clinical trials and in the approaches of reporting research parameters.