All generic medications must undergo certain tests to compare them to brand-name medications. The U.S. Food and Drug Administration (FDA) then looks at these tests to decide if the generics are equivalent to the brand-name medications and assigns each generic a rating. An "AB" rating means that the FDA has determined that a generic medication is equivalent to a brand-name medication. All of the generic nortriptyline versions currently available have an "AB" rating, meaning they should be equivalent to Pamelor.
However, generic medications are allowed to have different inactive ingredients than the brand-name medication. These might include fillers, dyes, or other ingredients that may cause problems for people with allergies or sensitivities.