I was diagnosed in 2000 and have had 3 day infusions of steroids only 4 times in all those years. The last time was 09, when I'd moved here to Spain wrongly assumed that I'd moved into SPMS and had run out of Rebif some 8 months earlier.
I had a very sudden and acute relapse which paralysed my right side.
I have hypertension (high blood pressure) which goes a bit mad if I have an exacerbation or a relapse, so I've been told to go to my neuro and he'll arrange steroid treatment without waiting - but fortunately I haven't had a relapse since then and my neuro asked for funding for me to restart Rebif again and to my surprise, the hospital said yes and (touch wood) I'm back to stability.
The steroids worked fast and my BP was down to normal within hours - it's a rare MS thing. My right side was better fast, but took some months to gain back the balance and control.
Hitting the relapse hard with a three day treatment shouldn't put you at risk of gaining weight. They'll monitor you well in hospital and do ask for Omeprazole for your stomach and something to help you sleep if can't - and mints for the metallic taste in your mouth.
I hope it all goes well,