NOTHING to declare. Most pharmacies will accept Pesos or USD. They are looking for Meth., Cocaine, and Fentanyl... not medication to stop you from being nauseated and throwing up.
"U.S. residents entering the United States at international land borders who are carrying a validly obtained controlled substance (other than narcotics such as marijuana, cocaine, heroin, or LSD), are subject to certain additional requirements. If a U.S. resident wants to bring in a controlled substance (other than narcotics such as marijuana, cocaine, heroin, or LSD) but does not have a prescription for the substance issued by a U.S.-licensed practitioner (e.g., physician, dentist, etc.) who is registered with, and authorized by, the Drug Enforcement Administration to prescribe the medication, the individual may not import more than 50 dosage units of the medication into the United States.
If the U.S. resident has a prescription for the controlled substance issued by a DEA registrant, more than 50 dosage units may be imported by that person, provided all other legal requirements are met. Note: Only medications that can be legally prescribed in the United States may be imported for personal use."
Seems fine up to 50 doses.