I'm not aware of any reasons spiny eels react badly to medicines. Moray eels, yes, but not spiny eels. Keep an eye on its breathing rate, and if something looks bad, do a 50%+ water change as soon as you can.
One thing I will mention is that spiny eels are very prone to bacterial skin infections, so you don't want the whitespot to get serious. In my opinion, keeping spiny eels in a tank with gravel rather than sand is a death sentence. Sooner or later the eel scratches itself on the gravel while digging, and the wound goes nasty.
Cheers,
Neale