Well, Lefty threw up twice this morning and I think it got out all of the plastic--there was a total of three chunks. There's been none in his poop. He's been a little sluggish this morning but just now he barked at the oil truck, which seems to indicate that he's feeling better. I think I'll feed him his regular dog food so that he can get back to normal quickly--the prescription dog food just seems to leave him hungry all of the time, so his nose is constantly to the ground looking for "snacks!" Thanks to all of you for your replies!