bill gates
Bill Gates.Reuters

Billionaire philanthropist Bill Gates has joined the ranks of those who believe the market for bitcoin is not a "good thing."

The world's second richest man on Tuesday focused on the downsides of Bitcoin and its peers, a significant shift from its previous stance in 2014, when he was quoted as saying that Bitcoin is "better than currency".

Gates criticized cryptocurrencies for their anonymity, a feature that allows currency holders to keep their identity discrete and thereby making it easier for illegal transactions to go unnoticed.

"The main feature of cryptocurrencies is their anonymity. I don't think this is a good thing," he said during an "ask me anything" discussion on the social media platform Reddit.

He also railed against cryptocurrencies for anonymous payments that help illegal drug trade deals.

"The government's ability to find money laundering and tax evasion and terrorist funding is a good thing. Right now cryptocurrencies are used for buying fentanyl and other drugs so it is a rare technology that has caused deaths in a fairly direct way."

Fentanyl, also known as fentanil, is an opioid which is used as a pain medication and together with other medications for anesthesia. It is a depressant drug, which means it slows down the messages travelling between the brain and body.

Gates added: "Yes - anonymous cash is used for these kinds of things but you have to be physically present to transfer it which makes things like kidnapping payments more difficult."

According to Gates, cryptocurrencies have made it easier to profit from certain types of crime as its easier to collect ransom without revealing one's identity.

The price of Bitcoin rose about 3.5 percent on Tuesday to $10,600.