Confirm change in dosage (8)
I believe the answer is:
diagnose
'confirm' is the definition.
(confirm a medical problem)
'change in dosage' is the wordplay.
'change' indicates anagramming the letters.
'in'+'dosage'='indosage'
'indosage' with letters rearranged gives 'DIAGNOSE'.
(Other definitions for diagnose that I've seen before include "Find cause of illness in San Diego" , "Examine and identify (problem)" , "Identify (an illness)" , "Analyse a problem" , "Analyse medically" .)