The American billionaire - who has turned his attention towards philanthropic work over recent years - made the eye-catching statement in reference to the way digital currencies, like Bitcoin, are being used to buy potentially harmful drugs, such as synthetic opioid fentanyl.
During an 'ask me anything' session on Reddit, Gates was quizzed about the impact of crypto-currencies and he replied: "The main feature of crypto-currencies is their anonymity. I don't think this is a good thing. The government's ability to find money laundering and tax evasion and terrorist funding is a good thing.
"Right now, crypto-currencies 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 crypto-currencies is super risky for those who go long."
Gates' long-time friend and fellow billionaire Warren Buffett previously claimed that the obsession with crypto-currencies will "come to a bad end" at some stage.
By contrast, Gates suggested in 2014 that Bitcoin "was better than currency".
Meanwhile, his comments on the Reddit session have been criticised in some quarters, with people accusing the businessman-turned-philanthropist of attempting to influence the market.