6.5 Product Selection & Generic Substitution

Drug Product Selection Under PA Law

When a prescriber writes a brand-name drug, PA law gives the pharmacist authority — and in most cases the obligation — to dispense a less expensive generic equivalent unless the prescriber or patient blocks substitution. The controlling rule is 49 Pa. Code § 27.18 (drug product selection), read together with the federal FDCA and FDA's Orange Book ("Approved Drug Products with Therapeutic Equivalence Evaluations").

AB-Rated Equivalence

FDA assigns a therapeutic equivalence code to each approved drug in the Orange Book. An "A" code means the product is therapeutically equivalent to the reference listed drug; the critical subcategory is "AB", which means bioequivalence has been demonstrated and is the basis for generic substitution. Codes starting with "B" are not therapeutically equivalent and may not be substituted. PA pharmacists may select an AB-rated generic for a brand-name prescription — the prescriber's brand-name prescription is, by default, permission to substitute an AB-rated equivalent.

"Dispense as Written" / "Brand Medically Necessary" Override

Substitution is not automatic in every case. The prescriber may prohibit substitution by writing "dispense as written" (DAW), "brand medically necessary", or words of similar meaning on the prescription. When the prescriber does so, the pharmacist must dispense the brand product as written. A patient may also request that the brand be dispensed, even if a generic is available; in that case the patient pays the cost differential. A prescriber DAW is the most common override scenario on the exam — be alert to whether the prescriber wrote DAW or whether the patient merely requested brand.

Mandatory Lower-Cost Pass-Through

PA requires the pharmacist to pass the savings from substitution to the patient and the payer. The pharmacy may not retain the spread between the brand's reimbursement and the generic's acquisition cost as a hidden mark-up. If the prescriber prohibits substitution, the patient is informed and the higher-cost brand is dispensed. If a generic is dispensed, the lower cost is reflected in the patient's copay and the third-party reimbursement. For Medicaid patients, the pharmacist must dispense the lower-cost equivalent consistent with the PA Department of Human Services' Maximum Allowable Cost (MAC) pricing list; products priced above MAC may require prior authorization or a DAW. Third-party payers also use MAC lists and generic incentives — substitution without passing the savings is a violation and may be a basis for payer audits and Board discipline.

Biologics and Biosimilar Substitution

Biologic drugs approved under a Biologics License Application (BLA) have a separate FDA equivalence pathway: biosimilars and interchangeable biologics. Only an interchangeable biosimilar — one that FDA has determined can be substituted without the prescriber's intervention — may be substituted at the pharmacy. A biosimilar that is not rated interchangeable may not be substituted; the prescriber must specifically prescribe it. PA follows the federal FDA designation. The pharmacist who substitutes an interchangeable biosimilar must communicate the substitution to the prescriber within a reasonable time and record the product dispensed (NDC, lot, expiration).

Narrow Therapeutic Index (NTI) Drugs

Some drugs have a narrow therapeutic index (NTI) — a small difference between the dose that produces a therapeutic effect and the dose that produces toxicity. Examples include warfarin, digoxin, levothyroxine, phenytoin, carbamazepine, lithium, and theophylline. PA does not categorically prohibit generic substitution for NTI drugs, but pharmacists are expected to use professional judgment: a patient stabilized on one manufacturer's product should generally be maintained on that product, and substitutions should be accompanied by patient counseling and monitoring. Many NTI drugs have AB-rated generics in the Orange Book, so substitution is legal; clinical caution and patient-specific considerations drive the decision.

Table — Generic Substitution Scenarios

Traps

• A DAW is prescriber-specific; a generic can be dispensed on a refill if a new prescription is written without DAW.
• "Substitution permitted" is the default; DAW is the exception. Many exam questions reverse this — read carefully.
• A "generic is unavailable" scenario (shortage) requires the pharmacist to dispense the brand and document; the patient may not be charged the generic copay if the brand was dispensed solely because no generic was available.
• The pharmacist may not substitute a different dosage form (e.g., tablet for capsule) or strength — substitution is only within the same dosage form, strength, and route.
• Substitution rules apply only to the drug product; they do not extend to switching between brand and generic devices, biologics that are not interchangeable, or different salts/esters unless rated therapeutically equivalent.