Time course for development of psoralen plus ultraviolet A erythema following oral administration of 5-methoxypsoralen

Conflicts of interest: none declared. Sir, Psoralen photochemotherapy (psoralen plus ultraviolet A radiation, PUVA) is an effective therapy for psoriasis and other dermatoses. Determination of the minimal phototoxic dose (MPD) prior to PUVA treatment is advisable as suggested by the British Photodermatology Group guidelines.1, 2 8‐Methoxypsoralen (8‐MOP) is widely used as the standard oral psoralen for PUVA. However, its use is limited by side‐effects including nausea, vomiting, pruritus and erythema. 5‐MOP is commonly used in those patients who develop gastrointestinal adverse reactions as it seems to be better tolerated. Ibbotson and Farr3 showed that peak erythema after oral 8‐MOP occurs at or beyond 96 h in 75% of subjects. Behrens‐Williams et al.4 and Grundmann‐Kollmann et al.5 demonstrated similar findings with bath, and cream and gel formulations of 8‐MOP. More recent studies by Man et al.6, 7 showed that MPD determination at 72 h underestimates the phototoxic response of PUVA, as peak PUVA erythema after 8‐MOP and 4,5′,8‐trimethylpsoralen occurs at 96–120 h.