Is it safe to use a dietary supplement after mono?

I'm just curious, I was using Oxy Elite Pro... It's not necessarily all about fat burning, its supposed to give you an energy boost and supposidly suppresses appetite. i've never felt a difference in less appetite but it has helped a little with energy. Mono completely diminishes you of energy, that I know. And I have never and will never starve myself. However I have a big event soon and I just want to look my best and oddly enough I've gained weight with mono. Suggestions? I don't have much time