I wouldn't worry. Personal prescription drugs are not a problem. In Spain, they come with a piece of paper explaining all the pertinent information about the medicine, which helps, but I know that's not the case in the States. But since there's a 99.99% chance you won't even be stopped, I wouldn't be concerned.
Customs in Spain, as a rule, is not very strict, especially if you're coming from the States. I don't want to go out on a limb and say they won't stop you, but after fifteen years I've never been stopped and I don't know anyone who has.
If you're really concerned I would suggest:
1) Getting a signed printed letter from your doctor stating what the medicine is and its purpose.
2) Checking with the consulate, but they aren't always very helpful.
But once again, these are simply super-safe measures. Personally I wouldn't even bother.