A friend of mine said that if I take a flight, I will need a statement written by a doctor that my migraine medicines (Pronaxen) are legal. Is that true? I have had flight around the Europe, like Helsinki, Riga, Copenhagen, Lisbon, Barcelona, but this question popped to my mind only now. How can a doctor make sure that I won't remove the medicine and put something illegal to the pot after he or she has been written the statement?
You should have written valid drug prescriptions with you when traveling to reduce the potential problems you can encounter when getting in/out of a country.
Some prescriptions drugs can be more "controlled" in different countries.
Valid drug prescription can be really useful in case you need to have a refill your prescriptions while traveling (for example in case of theft).
If YOU decide to substitute the drug with an illegal drug, then the burden will be yours and only yours, if you are controlled and the drug is tested.
Not all countries have the same rules about what is over-the-counter and what is prescribed. You need to look up your specific medication and perhaps ask the country you are going to.
For example, I use Sudafed for allergy and cold congestion. In Canada, you can buy it over-the-counter if it has acetaminophen added (which makes it harder to use as a starting point for meth-making) and can ask at the pharmacy counter for the single-ingredient version. In the US, it's the same except that you have to show ID at the pharmacy counter and they write your details down. But in New Zealand their Sudafed has a different ingredient (one that was tried briefly in Canada and just doesn't work for me) and the version I have is very strictly controlled. As in "you can't come into the country because you tried to bring that stuff" controlled. I have been exchanging emails with NZ customs for months working out the wording of my doctor letter, how much of the packaging I need to bring, and so on. I'm bringing a blister-pack version so it's more obvious that I'm bringing what I say I'm bringing.
Doing this level of research for every combination of medication and country is impossible. If your medication is already prescribed to you, just leave it in the original packaging and bring your prescription along with whatever paperwork the pharmacist gave you when filling the prescription. If it's over-the-counter, get in touch with the customs people of the country you're going to and ask if you need a doctor's letter. It's worth putting that time in to know you will be ok.