is this what you read? if it is, the way this is written sure seems to indicate you are in fact allowed to bring it in with a prescription if you follow all the rules that go along with it.
"It is illegal to bring into Mexico some over-the-counter medicines commonly used in the United States, including inhalers and some allergy and sinus medications. Specifically, products that contain stimulants (medicines that contain pseudoephedrine, such as Actifed, Sudafed, and Vicks inhalers) or codeine are prohibited.
Import medicines for personal use to Mexico
Passengers traveling with medications for personal use will be authorized to enter Mexican territory only if they have a medical prescription or a letter from their doctor that mentions the amount of substance necessary for the patient during their trip to Mexico, the amount that the patient will take to Mexico and the daily dose.
At the point of entry into Mexico, you must report to the customs authorities and present the medical prescription issued by the competent authority. Said document must bear the name of the doctor who authorizes the prescription, his/her signature, contact details (telephone, address) and professional registration. The amount of medication you carry during the trip should not exceed the amount needed for your stay in Mexico. The medical prescription must be translated into Spanish.
Medications must be placed in hand luggage, with their respective boxes and in transparent bags.
Before transporting medicines, we advise you to contact your airline."