The product -- known as "Crescina" -- is widely advertised and sold in pharmacies and cosmetic stores. One of its ads confusingly features football star Ronaldo, who used to shave his head but has now let his hair grow.
The alleged crime being investigated is commercial fraud as "Crescina" -- the name is a play on the Italian word for "growth" -- was being advertised in Italy as having hair-growing properties, prosecutors said.
Investigators said they had commissioned their own scientific research, pointing out that if it did work if would have to be classified as a "medicine" instead of a "cosmetic" and would therefore be subject to stricter controls.
"Crescina" is produced by the Swiss company Labo Cosprophar Suisse, which also makes supposedly anti-ageing products. It holds 75 trademarks and 17 patents and its Italian branch manages exports to 30 countries around Europe.