CHICAGO (AP) — Cancer patients’ gray hair unexpectedly turned youthfully dark while taking novel drugs, and it has doctors scratching their heads.

Chemotherapy is notorious for making hair fall out, but the 14 lung cancer patients involved were all being treated with new immunotherapy drugs that work differently and have different side effects. A Spanish study suggests that may include restoring hair pigment.

All but one patient had at least stable disease and responded better to treatment than other patients. That suggests that hair darkening might be an indication that the drugs are working. But the researchers say more study is needed to show their results aren’t a fluke, and to answer how the drugs might change hair color.

The study was published online this month in JAMA Dermatology.