Although standards to ensure strength, quality, purity, and bioequivalence exist for drug products, the standards allow for variations in certain factors that may produce other differences from product to product. Don't overlook the convenience of the package. The package must protect the drug in accordance with USP requirements, but packages can be quite different in their ease of carrying, storing, opening, and measuring. These inactive ingredients may vary from product to product.
Some states require that the pharmacist dispense exactly what is prescribed.
However, other states allow the pharmacist to dispense less expensive medicines when appropriate. Aside from differences in the drug product, there are many other factors that may influence the effectiveness of a medicine. For example, your diet, body chemistry, medical conditions, or other drugs you are taking may affect how much of a dose of a particular medicine gets into the body. For the majority of drugs, slight differences in the amount of drug made available to the body will not make any therapeutic difference. For other drugs, the precise amount that gets into the body is more critical.