The Food and Drug Administration (FDA) requires the makers of generic drugs to prove that their product has:
the same active ingredient as the brand-name drug
the same strength and dosage form as the brand-name drug
the same route of administration (how you take the drug)
The generic drug is also required to work in the same way and just as well as the brand-name product.
According to the FDA, the generic form of Gleevec meets these requirements. This means that the FDA guarantees that the generic form is as effective as the brand-name drug.