Numerous dog food brands in the US are being recalled after an investigation found traces in them of a drug normally used to euthanise animals.

Reports in the US have claimed at least 31 different varieties of dog food have been recalled after a drug used to put pets to sleep, pentobarbital, was found present during lab analyses.

According to Washington DC ABC affiliate WJLA, the majority of the products are owned by the company, Smucker's and include popular items like Gravy Train and Kibbles 'N Bits.

The news outlet said that out of 15 cans of Gravy Train tested, nine were positive for the drug.

Though the low levels were not thought to be lethal to pets, the drug is still not permitted.

In a statement on their website, Gravy Train said that "the presence of this substance at any level is not acceptable to us and not up to our quality standards" and apologised for the "concern this has caused".

"Out of an abundance of caution we initiated a voluntary withdrawal on specific shipments of Gravy Train® canned/wet dog food because they do not meet our quality specifications." The company said they had an internal investigation ongoing.

"We take this very seriously and are extremely disappointed that pentobarbital was introduced to our supply chain. We have narrowed the focus of our investigation to a single supplier and a single, minor ingredient, used at one manufacturing facility.

"We will take the appropriate steps to ensure this does not occur again," they added.

One expert that spoke to WJLA questioned where the drug would have come from if the company was not using euthanised animals in its food. In the statements on their websites, the company said they "do not use 3D or 4D meat such as dead, dying, disabled, or diseased animals."

The Food and Drug Administration has reportedly begun an investigation. Lists of the affected products can be found on company websites.