John Hudak is deputy director of the Center for Effective Public Management at the Brookings Institution. Aaron Klein is policy director of the Center on Regulation and Markets at the Brookings Institution. The opinions expressed in this commentary are their own.

With thousands of cannabis businesses popping up in states that have legalized marijuana for medicinal or recreational use, one important problem remains: banking access.

Since cannabis remains illegal under federal law — and the federal government oversees all US banks and credit unions — banks are often skittish about granting bank accounts to these businesses. The failure to find a policy solution at the federal level has created several economic challenges and costly unintended consequences. Congress needs to pass legislation that makes clear that banks can work with marijuana companies in states that have legalized it without fear of penalty or costly red tape.

Only one in about 30 banks or credit unions across the United States accepts a cannabis business as a customer. Those that do take on cannabis companies often charge them hefty monthly account and transaction fees, in part to help offset the extra costs they incur by doing so.

While there is no law that says banks can or cannot do business with cannabis companies, banks are required to file reports to Uncle Sam detailing a customer's suspicious or illegal activities. That can prove costly. A bank can be subject to large fines if it incorrectly reports on its transactions, or if a future bank regulator accuses it of not following the reporting guidance properly. The reporting can be extensive, often covering every single action a customer takes, as it is based on the premise that the illegal activity is happening underground and the money trail is necessary to find the criminals. One small credit union in Oregon that serves marijuana businesses filed 13,500 reports over the past two years for approximately 500 cannabis clients.

The cost of these reports are passed on to the marijuana companies — often at a premium — and then on to consumers. This can make it more difficult for sick patients to afford state-regulated medical marijuana products and can lead to higher arrest rates among people who can't afford to buy on the legal market.

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