Updated view of tars for psoriasis: what have we learned over the last decade?

Tars are one of the most effective, unknown, and oldest therapies for psoriasis. They include coal tar (CT) and biomass-derived products. These treatments, particularly the CT, have proven to be cost-effective with long remission times compared to other systemic or topical treatments. However, they have hardly evolved in recent years, as they are not well-embraced by clinicians or patients because of concerns regarding cosmesis and safety. This review summarizes current knowledge about the chemical characterization, mechanism of action, toxicity, and clinical studies supporting the use of tars for psoriasis over the last decade. Trends within these above aspects are reviewed, and avenues of research are identified. CT is rich in polycyclic aromatic hydrocarbons, whereas biomass-derived tars are rich in phenols. While the activation of the aryl hydrocarbon receptor is involved in the antipsoriatic effect of CT, the mechanism of action of biomass-derived products remains to be elucidated. No conclusive evidence exists about the risk of cancer in psoriasis patients under CT treatment. Large, randomized, double-blind, controlled clinical trials are necessary to promote the inclusion of tars as part of modern therapies for psoriasis.