If you are travelling to a Schengen Country and want to take medicines with you that fall under the Opium Act, you will need to have a Schengen Certificate or a Medical Certificate, depending on where you are going. Examples of such medicines are strong painkillers, sleeping pills, and drugs to reduce anxiety, such as Valium or Seresta , medication for ADHD or medicinal cannabis.
If you are going to a country outside the Schengen area you will in many cases need a Medical certificate.
If you take these medicines with you without going through the necessary procedures, you may be in violation of local law and risk a severe penalty. So you should always ask the Embassy of the Country that you are travelling to what the local requirements are.