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A Maximum Residue Limit (MRL) is an amount of residue that could remain in the tissue or food product derived from a food-producing animal that has been treated with a veterinary drug. This residue is considered to pose no adverse health effects if ingested daily by humans over a lifetime.
A Proposed Maximum Residue Limit is an MRL which is under consultation. Once the consultation process is complete, and scientific and safety issues raised if any are addressed, the proposed MRL will become an MRL.
The List of Maximum Residue Limits (MRLs) for Veterinary Drugs in Foods includes "no MRL required" status for some veterinary drugs in certain foods. The predicted levels of residues of these veterinary drugs in foods, under the approved conditions of use, are considered not to pose a risk to human health due to a variety of factors which include: the drug is of low toxicity or is generally recognized as safe, is poorly absorbed or bioavailable, is rapidly metabolized or eliminated, is likely infrequently used, is a normal component of the human diet, or represents a small fraction of endogenous production. The decision on the safety to the consumer is made based on the totality of the evidence, and is specific to the approved conditions of use in a species of animal. Consequently, for the protection of human health, there is no need to routinely monitor the residues of these veterinary drugs in specified foods.
Maximum Residue Limits are established by regulatory agencies for purposes of monitoring of residues of chemicals (veterinary drugs, pesticides, inorganic chemicals, etc.) in food. In the US, they are called tolerances and are established by the US Food and Drug Administration's (FDA's) Center for Veterinary for Veterinary Medicine (CVM).
While efforts are made to harmonize, our respective conclusions may sometimes result in establishing different MRLs or tolerances for veterinary drugs. In Canada, VDD bases veterinary drug marketing, labeling and the establishment of MRLs on scientific assessments that take into consideration scientific views from other jurisdictions. Canada's drug approval system, animal husbandry practices (care and management) and legislation vary from that of other countries, including the US. Food consumption patterns also vary according to cultural practices in different countries. There also may be differences in the methodology used to measure residues or in the way that the MRLs (tolerances) are calculated.
The health of Canadians is of paramount importance to Health Canada. VDD works to ensure the safety of foods derived from food-producing animals treated with veterinary drugs and that veterinary drugs sold in Canada are safe and effective for animals, and that the established MRLs pose no health risks to Canadians.
Once the evaluation of a veterinary drug is complete, stakeholders are consulted and have the opportunity to provide feedback on the proposed MRL. The proposed MRLs under consultation are available on Health Canada's website. After the period of consultation is complete, and the issues noted in the consultation have been appropriately addressed by Health Canada, a "Final Notice to Amend the List of MRLs for Veterinary Drugs in Foods" is posted on the Health Canada website, and the proposed amendments will be formally incorporated to the List of MRLs for Veterinary Drugs in Foods.
There are two possibilities why there are no MRLs published for some veterinary drugs for food-producing animals. Either there is no approval given for that particular drug or species in Canada or in cases where there is an approval, the process to establish a specific MRL has not been completed. In both cases, no detectable residues are permitted in food.