A spokesman for Pfizer, a pharmaceutical company which manufactures the real Xanax, said:
"We are alarmed by the rise of counterfeit Xanax and its growing availability on the internet and dark web.
"Counterfeits are difficult to distinguish from the real medicines and are extremely dangerous by their very nature given that they are not produced under regulated manufacturing conditions to ensure their safety.
"We have found dangerous ingredients such as boric acid, heavy metals and floor polish in counterfeit medicines.
"We will continue to work side by side with all law enforcement agencies around the world to help detect, disrupt and deter counterfeit medicines trade."