All generic medications must go through certain tests that 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 doxepin versions that are currently available have an "AB" rating, meaning they should be equivalent to Sinequan.
However, generic medications are allowed to have different inactive ingredients than the brand-name medication. This might include fillers, dyes, or other ingredients that may cause problems for people with allergies or sensitivities.