More than a dozen Spanish children have been diagnosed with hypertrichosis, a rare condition known as ‘Werewolf Syndrome’, after taking drugs thought to treat heartburn.

At least 17 children across the regions of Cantabria, Valencia and Granada were given the contaminated medicine, which was found to also contain a drug that treats alopecia.

The children began developing symptoms of the syndrome almost immediately, with parents noticing hair growing on their children’s foreheads and cheeks.

Further alarm was then raised after the unexplained production of hair extended to their arms, legs and hands.

One mother told Spanish newspaper El Mundo she had been feeding her 22-month-old daughter a syrup for a stomach complaint before noticing a change.

Another told newspaper El Pais her son started to develop “adult’s eyebrows.”

“It was very scary because we didn’t know what was happening to him,” she said.

"Werewolf syndrome," also known as hypertrichosis, is the excess production of hair, either in one specific area or all over the body. https://t.co/jVmCe2zmMM pic.twitter.com/BhXKMA1chN

— USA TODAY (@USATODAY) August 29, 2019

An investigation by the Spanish Agency of Medicines and Medical Devices declared Spanish pharmaceuticals company Farma-Quimica Sur was to blame for the mislabelling faux pas.

It found over-the-counter indigestion pills had been tainted with minoxidil – medicine used during hair-loss treatment – and children’s overconsumption of the alopecia-fighting drug resulted in the ‘werewolf syndrome’ outbreak.

The investigation also suggested the affected batch of omeprazole was imported from Indian firm Smilax Laboratories Limited.

Health officials have since prohibited the Spanish manufacturer from manufacturing, importing or distributing any drugs until it has concluded its full investigation into the incident.

The first cases were reported in early July, with contaminated batches pulled from pharmaceutical shelves.

As for the condition of the now werewolf-like children, it is expected to improve within weeks as their excessive hair growth begins to fall out.

‘Werewolf syndrome’ is typically a genetic condition, and it is unknown whether there have ever been any other similar random outbreaks across the world.