Yea, that's one of the issues. You have to go with smaller doses, hence why there is Bettafix. It's a fifth of the concentration (if I remember correctly).
Personally, I have used it on all of my betta issues and have had no problems. It helped my fin rot issue with Marlboro and helped with some damaged girls when I had a sorority (never doing that again...).
55gPLANTED:Rainbows/Loaches/Garras|30gVIV:Crestie|20gL:Leos|10g:Pearl Fighter|5g:Betta|5gChi:Spark Gourami/Badis|5g:ADF|2.5g:Betta|2.5g:Betta|