If you need to refill on the road (like if you lose your meds, or they are damaged somehow), then you should bring along the relevant perscription documentation, and possibly a letter from your doctor (if you can get this stuff in Spanish as well as English, it will help).
Towards the latter part of my Camino (in Sagahun, I believe), I visited a farmacia to get some ibuprofen (the Camino Wonder Drug). An English-speaking husband and wife were trying to get some perscription pain medication, and the farmacist was unable to help them because they had no documentation. The husband was apparently in a lot of pain due to an existing chronic condition. The farmacist spoke decent English and was very helpful to me, so I know he wasn't being obstinate towards them. They left unhappy, and I think their Camino was in doubt at that point.
With that in mind, I recommend researching what you'll need to get a perscription filled in Spain, and prepare accordingly. The Camino is the trip of a lifetime, and it would be a shame to blow it for lack of some routine paperwork. Buen Camino!
