At least 17 babies and children in Spain began growing hair all over their faces and bodies after they were accidentally given the hair-loss drug, minoxidil, which a Spanish pharmaceutical company had mislabeled as a medication to treat acid reflux.

The error lead the children to develop a form of the rare condition, hypertrichosis, aka “werewolf syndrome,” Spanish authorities reported.

One mother told Spanish news outlet El País how the drug affected her baby boy, who was just six months old when he began growing excess hair: “My son’s forehead, cheeks, arms and legs, hands became covered in hair … He had the eyebrows of an adult. It was very scary because we didn’t know what was happening to him.”

Another mother, whose three-month old was affected, told the outlet of the ‘“anguish of going from one doctor to another” to try to figure out what was happening. Some were initially told it could be a genetic condition or a metabolic disorder.

Meanwhile, investigators at the Spanish Agency of Medicines and Medical Devices tracked the problem to a factory run by the pharmaceutical company Farma-Química Sur SL, in southern Spain.

Investigators had homed in on syrups for acid reflux that are prepared as compound drugs, that is, drugs that are formulated on an individual basis by pharmacists. The syrups were supposed to include the drug omeprazole, a common drug to treat acid reflux. But according to investigators it appeared that when Farma-Química Sur distributed batches of what was labeled as omeprazole, they were actually distributing minoxidil, aka Rogaine, which is used to treat hair loss as well as hypertension. Investigators added that it wasn’t an issue of an impurity or that the drugs were mixed together. Rather, it was a labeling error.

Health authorities subsequently recalled the batches and the factory was closed over issues with “control standards.”

Health authorities said that the affected children should lose the excess hair in a matter of months after they stop taking the minoxidil syrups. However, it’s unclear if there will be other effects since minoxidil has not been studied in babies. One mother said her son showed signs of liver damage after going through two and half bottles of the syrup, though his doctors said they expected him to fully recover.

The public prosecutor of Cambria has opened a criminal investigation, and at least four affected families have filed criminal lawsuits against Farma-Química Sur.