In the province of Quebec, Canada, a new regulation restricting usage of antimicrobials of very high importance for human health (Health Canada category 1 antimicrobials) in production animals is effective since February 2019. The objective of this study was to estimate changes in AM sales in dairy herds after the implementation of the regulation. Following the implementation of the new regulation, sales of these antimicrobials decreased, without evidence of an increase in the use of lesser importance antimicrobials in dairies. The veterinary clinic was a strong predictor of the herd-level reduction in sales of very high importance antimicrobials. The results demonstrate the effectiveness of restrictive regulations to limit sales of antimicrobials of very high importance for human health in animal production.