U.S. regulators are concerned about the safety of cannabis compounds taken by animals leaching into human foods like milk and eggs.

The Food and Drug Administration on Wednesday issued a batch of warnings to companies that sell CBD – a non-psychoactive compound in marijuana – and Delta-8 THC, a knockoff of the substance that does get you high, THC. One of the letters went to a Hendersonville, North Carolina-based company called Kingdom Harvest, which markets “hemp extract” and CBD for livestock that can be slipped into the food bowls of horses, cows or alpacas – or given to them via peanut butter.

“In addition to raising potential concerns regarding safety for the animals themselves, CBD products for food-producing animals raise concerns regarding the safety of the human food (meat, milk, and eggs) derived from those animals,” the FDA said in its warning letter. The agency asked the company to immediately stop selling any unapproved CBD products for food-producing animals, while noting there’s not much data on whether CBD given to animals winds up in the food they produce.

Kingdom Harvest didn’t immediately return a message seeking comment.

The U.S. Department of Agriculture awarded a grant to Kansas State University two years ago to study whether feeding hemp to livestock leads to unacceptable concentrations in food supply. The university recently published a study showing that giving industrial hemp to Holstein steers reduces their stress levels.

This isn’t the first time that the FDA has issued such warnings. The latest letters, sent to four other companies, are part of the agency’s ongoing struggle to deal with unregulated products derived from cannabis plants. Wednesday’s warnings mostly focused on Delta-8 THC, which can be made cheaply with things like battery acid and contains contaminants. Delta-8 has already been banned in many U.S. states.