What to expect when deworming

Realized today that my cats have worms. My grey/black boy has had little white things on his bum for months, I just couldn't really figure out what they were till today. They don't vomit from it and there is not weight loss or anything.

Fiance picked up Profender today from the vet. Wondering what to expect from it. Are they going to vomit? Are they going to loss worms all over the place or will it be contained to the litter box?   

Also, should I disinfect their litter box after the treatment? Is it essential? I don't want to do it unless it is essential cause I just did it not too long ago.

Your little one might end up vomiting as a side effect of the medication,but probably won't vomit up worms, but, really, it just depends on what all types of worms he has.  It sounds like he has tapeworms from your description.  Does he have fleas?  If so, that's how he got the worms.  If so, are you doing anything to treat for fleas?  They go hand in hand.  Did your Vet discuss all of this with your fiance?  As to worms ending up everywhere, they shouldn't.  The medication should kill them and if they come out they would be in the litter box.  Did you get more than ONE dose.  Usually you have to dose them up to 3 times for it to be effective, I believe. 

As to disinfecting the litter box, this really should be done on a monthly basis anyway, so I would say it's a good idea to do it maybe with the next dose, which should be in two weeks.