The Mexicnn Federales raided Progresso this winter(2005) and closed a number of unlicensed pharmicies and I suspect some of them were selling phony repackaged drugs. We always try to price medications at the larger establishments.
Make sure you knw EXACTLY what you need and spell the drug name correctly since you DO NOT generally have to have a prescription and the counter personnell may or may not be registered pharmacist.
Always declare your prescription drugs as "Medicine" when asked by the Border Patrol agent.
They will sometimes inspect your drugs and have been know to ask for a prescription for narcotics. Most of the time, there is no problem.
Just DO NOT declare it as "drugs" since this will raise all kinds of flags.