Gates told a Reddit "Ask Me Anything" (AMA) session on Tuesday that the anonymous and unregulated nature of virtual currencies, which are decentralised and therefore do not answer to any government or central bank, means that they enable criminals to carry out black market peer-to-peer transactions, money laundering, tax evasion and other illegal financial exchanges.
The entrepreneur cited the example of fentanyl - a powerful pain medication - being bought online without proper supervision or oversight as evidence that people's lives were being placed in danger from the threat of overdosing.
"The main feature of cryptocurrencies is their anonymity. I don't think this is a good thing," he said.
"The Governments [sic] 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.
"I think the speculative wave around ICOs [initial coin offerings] and cryptocurrencies is super risky for those who go long," Gates added.
While the influential tech executive sees digital currencies' ability to sidestep conventional regulatory infrastructure as a negative, early adopters of electronic cash alternatives like bitcoin felt the opposite.
Bitcoin's idealistic cypherpunk originators, angered by the financial crash of 2008, saw it as a means of bypassing crony capitalist institutions in the same anti-establishment spirit of "taking back control" that inspired such movements as Anonymous, Occupy and WikiLeaks.
However, sites like Silk Road - which allowed illegal commodities to be bought and sold in cryptocurrencies - have since given the sector a notoriety it has found hard to shake off.
The lack of consumer protection cryptocurrencies offer and their vulnerability to misuse continue to make the likes of bitcoin a risky investment proposition in the eyes of many.
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