Differences in phototoxic potency should be considered when tetracyclines are prescribed during summer-time. A study on doxycycline and lymecycline in human volunteers, using an objective method for recording erythema.

In order to test the phototoxic potency of the two tetracyclines most frequently prescribed in Sweden, a double-blind cross-over study using a double-dummy technique with doxycycline 0.1 g twice daily, lymecycline 0.6 g twice daily, and placebo, was performed in 15 healthy volunteers. Drugs were given for 3 consecutive days, and on the third day volunteers were tested with 25, 50, 75 and 100 J/cm2 of artificial long-wave ultraviolet radiation (UVA), and assessed 6 h later for erythematous photoreactions. Objective readings were made using skin reflectance spectrophotometry. All three substances were tested in each individual at weekly intervals. Within 50, 75 and 100 J/cm2 of UVA, lymecycline showed a slight increase in erythema compared with placebo, but this was not significant (50 and 100 J/cm2), or was of low significance (75 J/cm2). However, with the same doses, doxycycline showed a substantial increase in erythema compared with placebo, which was highly significant. We conclude that doxycycline has a higher phototoxic potency than lymecycline, and this is in agreement with earlier in vitro experimental data. We recommend that therapy with doxycycline is avoided during summer-time, and during holidays in a sunny climate.