The vet prescribed Milbemax when my kittens were 9 weeks old. Johnny weighed in at a fairly average 800g for his age, but Bettie was a bit small at 650g. They didn't have worms at the time, as far as I know, so this was simply a preventative measure. The vet told me to use only half a tablet for each kitten and they were fine, no side effects. I deliberately gave them a little kitten milk afterwards which they love and that may have helped it go down. I think they are supposed to take another dose soon.

Since your affected cats are unrelated it's unlikely that they are both particularly sensitive or allergic to a common ingredient, so I reckon your vet prescribed too high a dose, like other people have suggested. Are they smaller than average for their age and breed?

I really hope your cats are doing better now.