Thuja plicata, also known as Western Red Cedar, has a history of traditional medicinal use among Indigenous peoples of North America, primarily for respiratory ailments, skin infections, and as a topical agent for various skin conditions. Reports of its use for psoriasis specifically are not well-documented in ethnobotanical literature; however, Thuja species (particularly Thuja occidentalis) have been used in traditional medicine systems, such as homeopathy and folk medicine, to address skin diseases, including psoriasis and warts. The rationale behind its application is generally based on its purported antimicrobial and anti-inflammatory properties derived from essential oils and other phytochemicals present in the plant.

Despite this traditional use, there is a lack of robust scientific evidence supporting the efficacy of Thuja plicata in treating psoriasis. Most studies and clinical trials focus on related species or their extracts, with only limited in vitro or animal research addressing mechanisms relevant to inflammation or skin health. There are also potential risks due to the presence of thujone, a neurotoxic compound found in Thuja essential oils, which raises safety concerns for both topical and internal use.

In summary, the use of Thuja plicata for psoriasis is primarily rooted in traditional practices, with minimal direct evidence and no strong clinical validation in contemporary scientific literature.