Just thought I'd mention that bettas are generally categorized with tetras, catfish and other 'sensitive fish' where med doses are concerned and, personally, I generally use a reduced dose when treating mine with almost anything but antibiotics.
It generally seems to work out well as far as clearing up any problems go, and reduces the risk of problems from reactions to the meds.
Of course, all of my tanks have fish in the sensitive category, so that's something I tend to wind up doing anyway...