The coding system for therapeutic equivalence evaluations allows users to determine whether FDA has evaluated a particular approved product as therapeutically equivalent to other pharmaceutically equivalent products (first letter) and to provide additional information on the basis of FDA's evaluations (second letter). Sample TE codes: AA, AB, BC FDA assigns therapeutic equivalence codes to pharmaceutically equivalent drug products. A drug product is deemed to be therapeutically equivalent ("A" rated) only if: a drug company's approved application contains adequate scientific evidence establishing through in vivo and/or in vitro studies the bioequivalence of the product to a selected reference listed drug. those active ingredients or dosage forms for which no in vivo bioequivalence issue is known or suspected. Some drug products have more than one TE Code. Those products which the FDA does not deem to be therapeutically equivalent are "B" rated. Over-the-counter drugs are not assigned TE codes.