Medicine makes first person sick (4)
I believe the answer is:
pill
'medicine' is the definition.
(I know that pill is a type of dosage)
'first person sick' is the wordplay.
'first person' becomes 'p' (ist letter of 'person').
'sick' becomes 'ill' (similar in meaning).
'p'+'ill'='PILL'
'makes' acts as a link.
(Other definitions for pill that I've seen before include "nuisance to put up with" , "Dreadful person" , "Medication" , "Medical tablet" , "Dose of medicine" .)