Heart medication immediately starts to induce nausea (6)
I believe the answer is:
statin
'heart medication' is the definition.
(type of heart medication)
'immediately starts to induce nausea' is the wordplay.
'immediately' becomes 'stat' (stat can informally mean at once).
'starts to' suggests taking the first letters.
The first letters of 'induce nausea' is 'in'.
'stat'+'in'='STATIN'
(Other definitions for statin that I've seen before include "Cholesterol-reducing drug" , "Drug given to reduce blood cholesterol" , "Drug reducing cholesterol" , "Cholesterol-lowering drug" .)