
I once went to a seminar, held by the manufactorer of Drontal. It was suggested that the vomiting that can occur, is caused by toxines created by the worms when they die. Sorry, can't be more precise, didn't take notes, and it was a couple of years ago.
I once gave the three girls I then owned their worming tablets at the same time, useing Drontal for the first time. Two hours later they all threw up on the stairs, coming down from the bedroom to get out into the garden

They obviously felt sick at the same time, after throwing up they were fine.
At the time I thought it was the Drontal tablets not agreeing with them, but the explanation of it being toxines is more likely.
HTH
Karen