Late wife was on serious pain meds. No problems crossing either way, on either trip. Be aware though that many of the common pain meds used in the US are not avail in Mexico and, if they are, they are extremely difficult to find. For example we needed Fentenyl (sp?) patches in PV and Guadalajara. No pharmacy carried them in either city...finally got them at a hospital pharmacy.
Buckeye: I would take a copy of your files from your Dr. Mexican Docs are reluctant to prescribe these meds without such documentation. I know you said you'll bring your own, but you never know.