How detailed does a travel letter for controlled medication need to be for customs purposes?

its hard to say without knowing the country. Most EU countries within the Schengen area dont need a huge amount of information. Some south east Asian countries need a very detailed letter about the medication, its timing and why the person is carrying it. Sometimes countries are as strict as to require information on how many tablets and the serial number of the boxes. Talk to your clincian about where you are going and what you are taking. Sometimesclincians may be able to provide a letter for you to discuss with a prescriber in your chosen country to help get a prescription of your medication the boundaries around import are particulary strict.