treatment onset

is what would be expected, but using onset would not be wrong.

Me, Doubt, I wouldn't say "onset," 'cause it's somewhat negative

http://onelook.com/?w=onset&ls=a

"Beginning" or "first day" seem pretty okay. One can't help wondering however, why he quit using it. Maybe after 5 days he had noted a slight improvement

Edited to comment, Aha! Rog might be right, if he had started treatment the first day it appeared

....so you can see, Doubt, how important context...

Some drugs were made to be taken in rounds. For example, prophylactic antibiotics are usually taken for 8 days.

The ointment contains a strong steroid, which may facilitate infections if abused.