No experience here, but here is what it says on the CBP site. It appears the 90 day thing is just guidance.
Non-U.S. Citizens Traveling with Medication
For non-U.S. citizens temporarily visiting the United States, you may be allowed to travel with your own medications.
In general, you should have with you a valid prescription or doctor’s note—written in English—to bring medication to the U.S. The medication should be in its original container with the doctor’s instructions printed on the bottle. If you don’t have the original container, bring a copy of your prescription or a letter from your doctor explaining your condition and why you need this medication. Travel with no more than you need for your personal use during your stay. A rule of thumb: Bring no more than a 90-day supply of medication.
If you’re staying longer than 90 days, you may have additional medication sent to you by mail or courier. You should be able to provide documentation showing that the medication is being sent for your own use while visiting the United States. This documentation may include: a copy of your visa and passport, a letter from your doctor, and a copy of your prescription (in English). See the FDA’s Regulatory Procedures Manual (RPM) Chapter 9-2 “Coverage of Personal Importation” for more information.
Warning: The U.S. Food and Drug Administration prohibits the importation, by mail or in person, of fraudulent or counterfeit drugs and medical devices. These include unorthodox "cures" for medical conditions such as cancer, AIDS, arthritis, or multiple sclerosis. Additionally, unapproved drugs have resulted in patient harm, and the FDA works to protect patients from the risks posed by these drugs. See this page for more information: https://www.fda.gov/drugs/enforcement-activities-fda/unapproved-drugs.
If you have questions regarding the import of medication, please refer to the FDA’s Personal Importation page and Frequently Asked Questions About Drugs. For additional questions, please email the FDA’s Division of Import Operations (DIO) at Imports@fda.hhs.gov or phone (301) 796-0356.
https://www.cbp.gov/travel/us-citizens/know-before-you-go/prohibited-and-restricted-items