"FNS is now reaffirming its long standing policy that a household may not utilize the SNAP medical deduction for the cost of any substance considered illegal under Federal law," she wrote. SNAP stands for Supplement Nutrition Assistance Program, also known as food stamps.
Silbermann called out violators, though not by name. 

"It has come to FNS' attention that some State agencies are allowing elderly or disabled households to deduct the cost of medicinal marijuana from their income for SNAP purposes," she wrote. 

The states -- which include Maine, Oregon and New Mexico -- had been allowing the deductions under a 2008 law that permits "allowable medical expenses" to be deducted for elderly or disabled recipients. 

Marijuana, though, does not qualify as allowable, Silbermann wrote, warning that states must "cease this practice immediately" or face penalties. 

USDA spokeswoman Alyn Kiel confirmed to FoxNews.com that the states have since discontinued the practice and come into compliance.