Sunscreens with a rating of SPF 100 may be more effective than ones rated SPF 50, according to a randomized trial, even though the Food and Drug Administration says that any SPF above 50 offers no additional protection.

The study, in the Journal of the American Academy of Dermatology, included 199 men and women given two tubes of sunscreen, one marked “right” and one “left.” They applied the cream to the indicated side of the face and neck, and then went about their normal activities on a sunny day at a ski resort. One of the tubes contained SPF 50 cream, the other SPF 100.

The next day, a dermatologist rated the severity of burn on a 5-point scale from none to most severe. On average, the score was more than twice as high for the SPF 50 side of the face as for the SPF 100.

The reason may be that in practice, people almost never put on enough sunscreen.

“If you look at the laboratory results, you don’t see much of difference between 100 and 50,” said the senior author, Dr. Darrell S. Rigel, a professor of dermatology at New York University. “But in the real world, the higher SPFs are much more forgiving, and since people are under-applying sunscreen, they’re much more likely to protect.”