“Proven prevention against poison plant rash,” claim ads for Ivy Block lotion. The bottle says it “helps prevent poison ivy, oak and sumac rash when applied before contact.” It’s been on the market since 1997, but at $14.99 for 4 ounces (six to eight applications), does it really protect against the itchy, red rash? Yes, concluded the Food and Drug Administration when it approved Ivy Block as an over-the-counter topical drug. But the dermatologists we consulted said you should still take other precautions.

When we asked to see the scientific evidence for the claims, the distributor, Hyland’s of Los Angeles, sent us a study published in the Journal of the American Academy of Dermatology in August 1995. In that clinical trial, involving 144 subjects with poison plant allergies, the active ingredient in Ivy Block prevented or significantly reduced the severity of skin reactions in patch tests with urushiol, the common allergen in poison ivy, oak, and sumac.

Ivy Block lotion appears to keep urushiol on the skin’s surface, preventing or reducing the amount that penetrates and produces the immune response, says Joseph Fowler Jr., M.D., a co-author of the study and a clinical professor of dermatology at the University of Louisville who is now a paid consultant to Hyland’s.

CR’s take: Ivy Block can reduce the risk of a rash, but when walking in areas where poison plants may grow, you should still wear long sleeves, long pants, and socks.