We have dealt with a similar issue also, and this is our typical approach. Do what works best for you.
1. Obtain letter and prescription from your home treating physician in English and Spanish, or whatever language is spoken, wherever you are traveling.
2 Keep copies of these documents with you in a plastic zip lock bag.
3. If possible, time your dosing so you need a minimum of the monthly/weekly doses for those types of sequenced doses.
4. If possible, plan your schedule so you have a dose before your departure from home and return trip home, and consider an extra dose to have available at a pharmacy, if appropriate.
5. Research physicians and clinics in a few locations on the Camino and in Santiago, so you have a quick and easily accessible resource at hand, if needed. Keep in the zip lock bag.
6. Consider purchasing travel "health insurance" if you are not covered when out of your home country by your health insurance that you use at home.
7. Inquire with your health insurance at home and/or your travel "health insurance", if they can assist you in advance of your trip and on your trip. Keep that information for future use.
8. Email a few physicians from your research list. If you speak Spanish, call one. Up front, offer to pay them for their professional time ( tha offer tells them that you are serious, value their time and expertise, and "greases your sled" with that physician), and ask to be able to call/email in the future. Present your situation, offer to send copies of your home physician's prescription and letter (English and Spanish translation), and ask if they will provide your prescription for use in Spain and through your EU transitting countries, and ask if they will email the prescription and allow you to pick up a paper copy when you pass through, or send the paper copy to a location in Spain that you prearrange where you can pick up the paper copy. Place copy of Spanish physician's prescription (email and paper) in the zip lock bag.
9. Keep the Spanish and your home physicians' contact information in zip lock bag.
10. Keep copies of documents, lists/links/maps of some clinics and pharmacies in easily accessible electronic media on your mobile phone.
11. Research this forum and continue to read the comments on your post.
12. After your Camino, consider posting a follow up note to inform and assist others as to how YOUR plan worked.
11. Enjoy your Camino. Do not fret. The Camino provides.
And this plan has worked for us in the past, reduced our trepidation and cooperation was great, especially when our 22 year old with a chronic condition traveled from the center of the USA to Perth, Western Australia.
We heard just what a medical parent wants to hear.
"Not a problem, we have it covered, here are my mobile and home numbers, call anytime".
And I am sure we have not heard all of the stories of that trip.
Really, what could go wrong with two just out of college young guys from the USA in Australia going to Australian rules football games, fishing, and barbies?
We didn't ask.
Buen Camino.
? Comments, other suggestions?