Isnt it so frustrating when children refuse to help themselves!
They often become so distressed from the pain or discomfort they are already in, coupled with their young age, to display any kind of reason with you..
We have an extremely stubborn toddler who wont take medicine, crys at the doctors/dentist etc...... We put it down to uncertainty on the childs part, illness is a break to their routine and causes anxiety as well as the physical symptoms that they are experiencing.
Luckily as he has got older, he will take "pink medicine" (calpol) we also now have a very patient dentist and he likes to go because the waiting room has toys and books for him to use.
http://www.netdoctor.co.uk/adhd/howdoigetmychildtotaketheirmedicine.htm
Have a look at this link and best of luck