Just like in the popular animated film Hotel Transylvania, children and babies in Spain started to grow dense layers of hair all over their bodies and faces, seemingly out of nowhere.

"My boy's forehead, cheeks, arms, legs and hands filled with hair. He had an adult's eyebrow," Ángela Selles, the mother of six-month-old baby Uriel, told the Spanish newspaper El Pais. "It was very scary because we didn't know what was happening," Selles said.

Parents and doctors were baffled by the sudden spike in hypertrichosis cases. It is also known as the werewolf syndrome and is characterized by excessive hair growth anywhere on a person’s body. Could this be a recessive condition passed down the family? Perhaps they had a bearded aunt somewhere among their ancestors they did not know about. Perhaps it was a metabolic disorder.

On Wednesday (Aug. 28), the Spanish Health Ministry finally announced that it had discovered the root of the problem. In a pharmaceutical mix-up, babies across Spain had been inadvertently dosed with medication for alopecia, or to put simply, hair loss.

The common denominator among the afflicted children: They were all taking the same medicated formula that was meant to contain omeprazole, a drug used to treat acid reflux disorders.

Barbara van Beck was an entrepreneur and celebrity who lived with a condition whereby her face and much of her body were covered with hair. She is believed to have had the condition hypertrichosis. "File:Barbara van Beck.jpg" by Zeromonk is licensed under CC BY-SA 4.0"

The drug was first identified in early July as a suspect for the condition and was recalled from stores. An ensuing inquiry by Spain's Agency for Medicines and Health Products (AEMPS) uncovered that rather than omeprazole, the formula the children were ingesting contained minoxidil, the active ingredient in Rogaine which is used to treat male-pattern hair loss.

This catastrophe was the result of a mix-up somewhere along the packaging process, where the minoxidil was mislabelled as omeprazole. By Aug. 6, 22 batches of the tainted formula had been recalled. On Tuesday (Aug. 27), the factory where the drug was produced closed down, El Pais reported.

Hypertrichosis is typically linked to an extremely rare genetic condition in which excessive hair growth begins in infancy and persists into adulthood. Only about 50 cases of this kind of hypertrichosis have ever been documented, according to JAMA Dermatology.

Thankfully, these kids will not be growing up into adult werewolves as the excess hair should start falling out within a few months. Nevertheless, it was a trying time for the parents that had to shuttle their kids to specialist clinics, and for older kids who were teased at school for their unusual appearance, The Telegraph reported. Also, at least one child developed liver damage as a result of the mix-up, according to El Pais.

Four families are now preparing to launch a lawsuit against FarmaQuimica Sur, the company behind the mixup.