I had a similar thing happen last week. I a partially chewed, empty box of Imitrex with no sign of the pills. I induced,vomiting in both of them, and one off hours visit to the vet, two X-rays and two days later, I found the untouched blister pack of pills.
My vet suggested buying small bottles of hydrogen peroxide and giving 50cc to start. She said once the bottles have been opened, they lose their fizz and effectiveness.