Of 30 volunteers, ten were given demethylchlortetracycline (600 mg/day), ten, doxycycline (200 mg/day), and ten, a placebo, for one week, and then all were taken on a voyage designed to maximize sun exposure. Nine out of ten receiving demethylchlortetracycline developed photosensitive eruptions, two of ten receiving doxycycline were also positive; while one volunteer on placebo was doubtful. These results support the concept of demethylchlortetracycline as a powerful phototoxic agent and suggest that doxycycline in the doses used will cause a lower incidence of this annoying side effect.
Blank H, Cullen SI, Catalano PM. Photosensitivity Studies With Demethylchlortetracycline and Doxycycline. Arch Dermatol. 1968;97(1):1-2. doi:10.1001/archderm.1968.01610070007001