What devastating news! Your love will get sweet Onyx through what may be ahead. Have you considered writing to your state's veterinary board about this? The vet should have told you about possible side effects before administering the medication. From what other PTers have posted, this seems to be the norm! Vets need to understand the importance of pets in the lives of those of us who love them!
Prayers continue to be prayed for Onyx, and plenty of lovebug being sent.....