Determine change in dosage (8)
I believe the answer is:
diagnose
'determine' is the definition.
(I've seen this before)
'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 "Identify (an illness)" , "Analyse medically" , "Determine cause of illness by analysing symptoms" , "Determine the nature of a medical problem" , "Establish nature of illness after study of symptoms" .)