Most rats and mice carry myco, but when stress or other factors are included in the picture, they can sometimes have a myco flare up. Jeanne, I was thinking it's Tetramycin? But yeah, like she said, it is light-sensitive and you should cover the bottle with foil or duct tape, something to keep the light out. This is all just in case you have a myco flare up. Our hairless rats get a little sneezy sometimes, but I just usually get some children's liquid benadryl(bubblegum flavored) and give them a drop. The sneezing and sniffling usually stops within the day. I hope all goes well with your new mice, and no, you shouldn't have to gas them, they should be ok. Just watch them and make sure they don't show signs of anything nasty