Which phrase prohibits substitution of medications?

The phrase that prohibits substitution of medications is "May Not Substitute." This phrase is typically used on prescriptions to clearly indicate that the prescribed medication must be dispensed as written, without substitutions for generic or alternative products. When a pharmacist encounters this directive, it signifies the prescriber’s intent that the specific brand or formulation is necessary for the patient’s treatment. It helps to ensure that the patient receives the exact medication that the healthcare provider deems most appropriate for their condition, which is essential in managing safety and efficacy.

In contrast, the other options don't carry the same directive about substitution. "Patient Must Approve" suggests that the decision for substitution relies on patient consent rather than a prohibition by the prescriber. "Product Not Available" indicates a logistical issue in fulfilling the prescription and doesn't inherently prohibit a substitution; it simply states that a particular item cannot be provided. Lastly, "Always Generic" implies that generics are preferred, which doesn’t address substitution restrictions at all.