When my cats have had any procedure done that requires pain meds, we usually get Buprenex liquid from the vet to squirt in their mouths. It makes them a little drunk, but helps with the pain. You might want to ask your vet about that. They got 0.3ml by mouth 2x a day.
From the Web:
Buprenex is used for:
Treating moderate to severe pain.
Buprenex is a narcotic analgesic. It works by working in the brain and nervous system to decrease pain.
Side Effects of Buprenex in Cats
*Buprenex is the brand name of buprenorphine, a synthetic opiate used to treat pain in cats. It is also commonly used as a pain medication in humans and dogs. Buprenex is injected into the system, where it is absorbed within 15 to 30 minutes. The effects can last as long as 8 hours. Before deciding if Buprenex is right for your cat, make yourself aware of the side effects.
Reactions With Other Medications
*Buprenex can interact with other medications that your cat is taking. If your cat is on barbiturates, tranquilizers, antihistamines or monoamine oxidase inhibitors, consult with your veterinarian on how the Buprenex might affect your cat.
Sedation
*Cats who have been injected with Buprenex often seem drowsy while the drug is in effect. Buprenex is an opiate, and cats who are injected will be less responsive or inattentive. The cat might also sleep heavily while the drug is its system.
Drop in Heart Rate/Blood Pressure
*Because Buprenex is an opiate and works through binding to the body's opiate receptors, it depresses some body functions. When a cat is on Buprenex, it will likely have a slower heart beat and lower blood pressure.
Slow Breathing
*Buprenex may affect the respiratory system. Cats tend to exhibit slower breathing when they are on Buprenex. The drug is not recommended for cats that have respiratory issues.