Bringing stronger medication into Thailand rules and policy

This isn't an immigration subject and hope its ok, so my mother a UK citizen wants to travel to Thailand
with myself and my mother has a prescription for patches containing Morphine and the actual type
of Morphine is Fentanyl . I think a letter from the doctor is a good idea and if anyone can add anything
about needing to get a permit and what happens when you land in BKK ?
Thanks in advance .

Travel advice from Gov.uk
https://www.gov.uk/foreign-travel-advic ... and/health
If it helps. It does mention certain medicines….

Just to let you know and anyone else , I had a look on a few web pages and the links provided wasn't very good also the Thai embassy in London wasn't very helpful so I just sent an e-mail to the Thai FDA and they were fine with it . I told my mum to bring a letter and keep medication in hand luggage and we never had any issues and I was worried if we didn't say anything it might cause a lot of hassle if checked so thought the best policy is honesty and it paid off .