Photocarcinogenicity of psoralens used in PUVA treatment: present status in mouse and man. Young A R Journal of photochemistry and photobiology. B, Biology There is good evidence that 8-methoxypsoralen is photocarcinogenic in psoriatic patients undergoing long-term photochemotherapy (PUVA) in the U.S.A. However, this conclusion has not been supported by two major European studies which have indicated that PUVA is a tumour promoter of damage initiated by other agents. Variation in PUVA treatment protocols in the U.S.A. and Europe may partly account for the different conclusions. There is much interest in the therapeutic potential of monofunctional psoralens. It is hoped that these may reduce long-term risk. Monofunctional and cross-linking psoralens have been shown to be photocarcinogenic in mouse skin. The relative risk of different compounds may be assessed in the mouse, but it is important to base comparisons on dose protocols that have been shown to be therapeutically effective.