The Microsoft founder also warned that another financial crash in the near future is "a certainty"
Microsoft founder Bill Gates has criticised cryptocurrencies such as Bitcoin, saying they have 'caused deaths in a fairly direct way'.
Mr Gates made the comments during a Reddit 'Ask Me Anything' (AMA) session to promote the work of the Bill & Melinda Gates Foundation.
He argued that cryptocurrencies are often used for buying illegal drugs on the so-called 'dark web'.
Questioned about his views on such currencies, the billionaire responded: "The main feature of cryptocurrencies is their anonymity. I don't think this is a good thing.
"The Governments 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."
He added: "I think the speculative wave around [Initial Coin Offering] and cryptocurrencies is super risky for those who go long."
The value of Bitcoin has fluctuated wildly in recent months - with the price of a single 'coin' reaching a peak of around $20,000 in December, almost 20 times the value at the start of 2017.
The price is currently at around the $10,000 mark, according to the CoinDesk website.
During his Reddit session, Mr Gates was also asked whether he thought there could be another financial crash in the near future.
He answered: "Yes. It is hard to say when but this is a certainty. Fortunately we got through that one reasonably well.
"Despite this prediction of bumps ahead I am quite optimistic about how innovation and capitalism will improve the situation for humans everywhere."
Gates - still regularly ranked as the world's richest person - stepped down from day-to-day operations at Microsoft a decade ago, and is now focused primarily on his philanthropic efforts.