Category D (“Prudence”) is the lowest risk category. While the risk to public health associated with the use in veterinary medicine of substances included in this category is considered low, a number of the substances in this category are listed as WHO CIAs (aminopenicillins, natural penicillins and isoxazolylpenicillin). It is acknowledged that these antibiotics are not devoid of negative impact on resistance development and spread, in particular through co-selection. Therefore, while there are no specific recommendations to avoid use of Category D substances, there is a general recommendation that responsible use principles should be adhered to in everyday practice to keep the risk from use of these classes as low as possible. Unnecessary use and unnecessarily long treatment periods should be avoided, and group treatment should be restricted to situations where individual treatment is not feasible.