Neither with sunlight nor with intense long ultraviolet LUV radiation was it possible to produce phototoxic reactions with orally administered doses of methoxsalen (8-methoxypsoralen [8-MOP]) customarily used to treat vitiligo and to promote tanning.
The optimal time of exposure was two hours after ingestion. A single dose was as effective as the same dose given daily.
Hot quartz ultraviolet lamps should not be used. A suitable source of LUV light is a bank of four black lights placed close to the skin. With 45 minutes of exposure at 6 inches, 40 mg of methoxsalen will consistently produce a phototoxic response. The dose may be raised to 60 mg as required.
Kligman AM, Goldstein FP. Oral Dosage in Methoxsalen Phototoxicity. Arch Dermatol. 1973;107(4):548-550. doi:10.1001/archderm.1973.01620190024005