With a rally of more than 400% over the past year, bitcoin has become increasingly mainstream, and everybody including prominent investors and policy makers have been talking about it. Elon Musk's Tesla recently invested US$1.5-billion in the cryptocurrency and said it would accept it for payments.

For Gates, it's not something Main Street should buy into — plus it's bad for the environment as mining the coins requires a lot of energy.

Elon has tons of money and he's very sophisticated, so I don't worry that his bitcoin will sort of randomly go up or down," Gates said in an interview with Bloomberg Television. "I do think people get bought into these manias who may not have as much money to spare. My general thought would be that if you have less money than Elon, you should probably watch out."