A group of cancer patients’ grey hair has unexpectedly darkened after they took new types of drugs, researchers have revealed.

Chemotherapy is known to make patients’ hair fall out, but the 14 people involved were all being treated with new immunotherapy drugs that work differently and have different side effects from chemotherapy. A Spanish study suggests those may include restoring hair pigment, at least in patients with lung cancer.

Noelia Rivera, a dermatologist at Autonomous University of Barcelona, said they thought it could be an isolated case when it happened with the first patient. But the research team found the same thing when they asked other patients for photographs of themselves from before treatment.

The 14 people were among 52 patients with lung cancer being followed to see whether they developed bad side effects from the drugs — Keytruda, Opdivo and Tecentriq.

While most patients did not have a hair colour change, the 14 cases suggest it is not an isolated finding. In 13 patients, hair turned darkish brown or black; in one patient, it turned black in patches.

The same drugs have been linked previously with hair losing colour in patients with another cancer, melanoma.

All but one of the 14 patients in the Spanish study responded better to treatment than other patients, suggesting that hair darkening might be an indication that the drugs are working, the researchers said.

Rivera said they were continuing with the study to search for an explanation.

“It’s a fascinating report – one of those things that comes out of the blue,” said June Robinson, a Northwestern University research professor in dermatology. Robinson is also editor of the medical journal JAMA Dermatology, which published the study online this month.

She said the results deserved a deeper look but cautioned that it was too soon to suggest that they might lead to new treatments for unwanted grey hair.

Rivera noted that the drugs used in the study had serious side effects that made them unsafe for healthy people. But if it is confirmed that they do change hair colour, a different drug could be developed to treat grey hair, she said.

The pharmaceutical industry has previously capitalised on unexpected drug side effects. Examples include the male pattern baldness drug Propecia, the eyelash growing drug Latisse, and Botox anti-wrinkle injections. Active ingredients in these drugs were initially approved to treat enlarged prostates, eye pressure problems, and eye muscle spasms.