What a generic must share with its brand, and what can differ

A search for lisinopril on DailyMed¹, the National Library of Medicine's public database of drug labels, returned 376 labels on Nov. 3, 2026. The count includes combination products, and it reflects how many companies make, package or relabel one of the most common blood pressure medicines in the country. Each label lists its own mix of fillers, binders and colorants.

All of the single-ingredient versions contain the same active drug in the same strengths. FDA's rules decide which features of a generic have to match the brand it copies and which are allowed to vary, and that line explains nearly every difference a patient notices at the pharmacy counter.

Generics now carry most of the volume. In Medicare Part D in 2022, 94.6 percent of fills for drugs that had a generic alternative were filled with the generic, according to a 2024 issue brief³ from the HHS Office of the Assistant Secretary for Planning and Evaluation (ASPE).

The features a generic must share

A generic reaches the market through an Abbreviated New Drug Application, which leans on FDA's earlier finding that the brand is safe and effective. In exchange, the applicant has to show sameness. The federal regulation on what an ANDA must contain⁴ requires evidence that the generic has the same active ingredient, route of administration, dosage form and strength as its reference drug.

Labeling is covered by the same rule. The proposed label "must be the same as the labeling approved for the reference listed drug," the regulation says, except for a short list of permitted changes, such as a different manufacturer name or an indication carved out because a patent still protects it. That is why a generic's package insert usually reads almost word for word like the brand's.

Quality rules apply equally. FDA says in its generic drug facts⁶ that generic makers must meet the same manufacturing standards as brand makers, and that it inspects their plants to check. The ANDA review process covers every site that makes, tests or packages the product.

How bioequivalence is measured

The last requirement is the one that does the scientific work. Under FDA's regulatory definition⁵, bioequivalence is "the absence of a significant difference in the rate and extent" to which the active ingredient becomes available at the site of drug action, when given at the same dose under similar conditions.

A typical study gives healthy volunteers the generic and the brand on separate occasions and measures the drug in their blood over time. Two numbers carry most of the weight. AUC (area under the curve) reflects total exposure. Cmax is the peak concentration.

To pass, the 90 percent confidence interval for the ratio of generic to brand, for both AUC and Cmax, must fall entirely inside 80.00 to 125.00 percent, as an FDA pharmacologist set out in a 2022 introductory presentation on bioequivalence⁸. The window is easy to misread. It does not let a generic deliver 20 percent less or 25 percent more drug. The generic's average has to sit close enough to the brand's, with low enough variability, that the whole interval stays inside the limits. Our explainer on the 80-125 rule works through the arithmetic.

What is allowed to differ, and why

Inactive ingredients

Inactive ingredients, or excipients, are everything in a tablet or capsule besides the drug: fillers, binders, coatings, dyes, flavorings and preservatives. They hold the product together, keep it stable and control how it breaks apart. A generic can use a different set from the brand, and two generics of the same drug can differ from each other.

FDA's position, set out in its questions and answers on generic drugs⁷, is that in most cases these differences do not matter clinically. The agency also acknowledges that in a minority of patients an inactive ingredient may affect tolerability or absorption. For someone allergic to a particular dye, the label of the exact product is what counts. FDA keeps a public Inactive Ingredient Database¹⁰ of excipients used in approved products, updated quarterly, though it does not say which product contains what.

Appearance

Generics usually look different from the brand, and the reason is legal. FDA's consumer publication on generics explains that US trademark law does not allow a generic⁹ to look exactly like other drugs already on the market. Appearance can also change between refills of the same generic when the pharmacy switches suppliers, a pattern covered in why your pill changed color. Each labeler also gets its own National Drug Code, so the code on the bottle changes with the maker.

Why the price gap is so wide

Generic makers skip the brand's clinical trials and spend little on promotion, but competition does most of the work on price. FDA's fact page⁶ says a single generic competitor can lead to price reductions of about 30 percent, and five competitors to drops of nearly 85 percent. An FDA analysis published in December 2019¹¹, using average manufacturer prices for drugs that first went generic in 2015 to 2017, found that a market with one generic producer had prices 39 percent below the pre-generic brand price, and one with five producers about 86 percent below.

The federal pharmacy cost survey shows the long tail of that process. GxRadar's analysis of CMS NADAC files¹² through October 2026 puts the average pharmacy cost of a generic atorvastatin 20 mg tablet at 18.3 cents in November 2013 and 2.8 cents in October 2026. Our price pages carry the same series for 47 generics, and a separate article covers the competition research in more depth.

How to look up one product's ingredients

DailyMed publishes the labeling companies submit to FDA. On Nov. 3, 2026, its home page² listed 158,962 labelings. GxRadar has a fuller walk-through in how to look up a drug on DailyMed. Each entry includes the full prescribing information and a structured table of ingredients and appearance.

1. Search by drug name. A common generic such as lisinopril returns hundreds of entries, one or more per labeler.
2. Narrow to your product. Match the labeler named on the pharmacy label, or search by the NDC printed on the bottle.
3. Read the description section. Labels list inactive ingredients there, often strength by strength. One generic lisinopril label lists mannitol, dibasic calcium phosphate, starches, colloidal silicon dioxide and magnesium stearate in every strength, with yellow ferric oxide in some strengths and red ferric oxide in others. The iron oxides are colorants, which is why strengths of one product can be different colors.
4. Check the ingredients and appearance table. It lists the active ingredient and strength, each inactive ingredient, and the tablet's color, shape, size, score and imprint code.

Whether two products can be swapped at the pharmacy is a separate question, answered by the therapeutic equivalence code in FDA's Orange Book¹³. A code starting with A means FDA considers the product substitutable for its reference; our guide to therapeutic equivalence codes explains the letters. FDA publishes Orange Book changes in monthly cumulative supplements¹⁴, according to its preface. Anyone who reacts to a specific ingredient can ask a pharmacist which manufacturer's version they are getting and whether another version is stocked.