Which abbreviation would indicate that a generic substitution is not permitted on a prescription?

The abbreviation that indicates that a generic substitution is not permitted on a prescription is "DAW," which stands for "Dispense As Written." When a prescriber includes "DAW" on a prescription, it explicitly instructs the pharmacist to provide the exact brand-name medication as prescribed, without substituting it for a generic equivalent. This directive is important as it ensures the patient receives the specific formulation that the prescriber deems most appropriate for their treatment, possibly due to differences in efficacy, safety, or patient response between the brand and generic products.

The other options do not convey the same meaning regarding substitution. "NS" generally indicates "no substitution," which might imply a similar intention but is not the standard abbreviation used in practice. "Rx" simply denotes a prescription and does not provide any information regarding substitution, while "PRN" means "as needed," which pertains to dosing rather than substitution status. Therefore, "DAW" is the correct and recognized indication that generic substitution should not occur.