Some banks and credit-card companies have begun restricting customers’ purchases of bitcoin, shutting down a popular way to buy the volatile digital currency.

This month, Capital One Financial Corp. decided it would no longer let its customers use credit cards it issues when buying bitcoin or other cryptocurrencies such as Ethereum “due to the limited mainstream acceptance and the elevated risks of fraud, loss and volatility.” The bank said it would “regularly evaluate the decision as cryptocurrency markets evolve.”

The firm’s decision followed Discover Financial Services ’ move in 2015 to effectively prohibit purchases of digital currencies with its cards.

Bank of America Corp. allows bitcoin purchases with the credit cards it issues. “At this point there is nothing that would block a transaction, but we are carefully reviewing our policy,” said a bank spokeswoman.

Citigroup Inc., which allows bitcoin purchases with its credit cards, is also reviewing its policy, according to a person familiar with the matter. TD Bank, the U.S. unit of Toronto-Dominion Bank , said that as a result of security measures some bitcoin transactions aren’t being processed.