Kevin Sabet, former Senior Policy Advisor to President Obama's Drug Czar, Gil Kerlikowske, and currently a consultant to drug prevention and policy organizations, said financial institutions that deal with medical marijuana organizations should be on alert.
"Smoked marijuana remains illegal in all states, and federal law -- while recognizing certain components of marijuana as having medicinal value -- does not allow the whole marijuana plant to be smoked for any purpose, including purported medical purposes," he said.
Sabet also warned that all states with legalized medical marijuana should pay attention to the enforcement actions in California.
"Remember, all actions have to be approved by Attorney General Holder, so it's hard to imagine that California would be the only place the Department of Justice is focusing on," Sabet said.
While dispensaries outside California have not received similar letters from the Justice Department, some have been audited by the IRS for taking business deductions that were related to "trafficking in controlled substances." The IRS can penalize cannabis dispensaries based on section 280E of the tax code, passed during the Reagan administration in 1982, which prohibited drug dealers to take any deductions based on trafficking activities.
Jill Lamoureux, chairwoman of the National Cannabis Industry Association, said she knew of one audit in Colorado, the details of which are confidential. She said the deductions were accepted an no additional taxes or fines were assessed.
"If the IRS determines across the board that this industry cannot take standard business deductions it will severely limit the ability of these businesses to thrive serving patients and contributing to our state's economies desperately in need of new growth industries," Lamoureux wrote in an email to ABC News.
Sabet said the federal government is sending a message that the "rush" in the medical marijuana industry is "over."
"People - including drug dealing organizations - flocked to the promise land of California thinking they could get rich off of this grey market, but the federal government is now reminding folks that there's nothing grey about marijuana markets," Sabet said. "'Marijuana is an illegal drug and we take it seriously,' is the message they seem to be sending."