A topic in Clinical Evidence summarizes the results of 2 small cross-over RCTs on the prevention of recurrent herpes labialis. The first RCT (n=38) found that sunscreen reduced recurrence at 6 days (0/35 vs 27/38). The second RCT performed in experimental laboratory conditions found that sunscreen reduced recurrence at 6 days (11/19 with placebo vs 1/19 with sunscreen).
Comment: The quality of evidence was downgraded by sparse data and some uncertainty of directness.