Study 2 examined the effect of repeated drug exposure and involved an induction period ( 6 repeat phases at the same body sites during which clindamycin/tretinoin gel and vehicle gel patches were applied for 24 hours , removed and sites irradiated with UVB + / - VIS ) , followed by a rest period ( 10 to 17 days ) , then a challenge period that used the protocol described for Study 1 .